Building Cajons – Attaching the Back Panel Without Clamps

This method is a fast, easy way to get a tight, fully sealed back attached  to your cajon build.  It works well with any back material. No clamps are used. Here’s what you need:

  • camping pad foam material, must be compressible foam of some type, about 1/2 inch (1.3cm) thick
  • a small amount of hot glue or similar to attach the foam to the back plate
  • 12 small blocks of plywood or wood,  (about 4x1x1 inch {10×2.5.2.5 cm} depending on the thickness of your back material)
  • a drill and screws for screwing down the blocks
  • some heavy objects to weigh down the box, total about 100 lb (45kg)

This method works well even with surfaces that are not quite perfectly flat.  Clean-up is minimal. Glancing at the photos below might be enough information to get you going.

In these photos I use polycarbonate for the cajon back but the method is the same for plywood. There’s some polycarbonate specific info at the end of the blog for those of you who want to walk on the wild side.

I cut and sand the back slightly larger than the box by about 1/16 in (1.5mm), then, after it is attached to the box, sand the edges flush.

Hot glue secures foam to edges of polycarbonte used for cajon back.
Use hot glue to attach narrow strips of camping pad foam along the edge of the OUTSIDE surface of the back.

Cut the foam strips to be an 1/8 inch larger than the box side thickness.  Usually about 1/2 inch to 1 inch (1.3 cm to 2.5 cm). The camping pad foam strips serves two purposes. It prevents the back from moving and it provides a compressible surface to distribute pressure evenly throughout the attachment zone. Do not use a full sheet of camping pad foam, it will cause the back to bow in the center and you won’t have a ‘stretched’ back.

Cajon construction using a polycarbonate sheet as the back. Blocks in place to hold the cajon box.
Turn the back over, the back goes face down with 6 small blocks offset from the edges. The foam supports the back. It is the only thing supporting the back.

The first tier of blocks must not touch the edges of the back. Leave them back about 3/8ths inch (1 cm). If you use wood blocks you’ll need to pre-drill holes for the screws or the blocks will split. I prefer plywood blocks. The first tier must be high enough so the second tier will not touch the adhesive oozing from the joint.

Have your second tier of 6 blocks ready to go. Once the adhesive is on you must work quickly. Use your favorite adhesive, in these pictures I’m using clear 2 part epoxy but for a wooden back I’d probably use Titebond III (not hot glue, that’s just for the foam to back temporary bond).

Apply a thin layer of adhesive around the outer edge of the cajon box first. This will give it time to soak in a little.
Apply a layer of epoxy to the edges of the polycarbonate.
Apply a thin layer of adhesive to the edges of the back.

Once your adhesive is on, turn the box over and carefully place it exactly on the back, aligning the edges.

For 15 or 20 seconds, slowly press the box down while slightly shifting its position to spread the adhesive evenly. Keep watching the edges, realigning the box as necessary and press down more. The box will slip around a lot at first and you must go slowly in the first few seconds to minimize slippage.

Place about 1/3rd of the weights on the box to keep it stationary and then remove tape if you taped the edges. Check to see if the box has moved and reposition if necessary.

Work your way around the box screwing in the second tier of blocks atop of the first. The second tier fits snug against the box. The second tier must be high enough that the adhesive oozing out of the joint never touches the upper block.

The lower tier of blocks is offset so as to not contact the adhesive. The upper tier of blocks hold cajon box in place.
The lower tier of blocks is offset so as to not contact the adhesive. The upper tier of blocks hold the cajon box in place.

I realign the box exactly on the back before securing each upper block. Once all the blocks are all in place, the box cannot move.

Turn the box over and carefully place it exactly on the poly, aligning the edges.
The box and back with the lower and upper tiers of blocks in place.

Once all the blocks are in place add the remaining weight. I use a lot of weight. Some of my early attempts with less weight were less successful. If you want, you could calculate the optimum amount for your adhesive because all the parameters are easy to measure.

Using big blocks to weighing down the cajon box while gluing the back on.
Weighing down the box. These are sitka spruce blocks which are fairly light. Total weight is about 110 lbs (50kg)

With the weights in place, reach inside the box, through the sound hole and scrape out the access adhesive that oozes from the joint. Then wipe the area with the appropriate solvent. The more excess adhesive you can remove the easier clean up will be after it has set.

Next day remove the weight and sand the edge to make the back flush.

The edge of the back sanded flush with the sides and top of the cajon.
The edge of the back sanded flush with the sides and top of the cajon.

A tight box is critical for good sound and durable construction. While building my first cajons I used clamps to attach the back. They worked but it was messy and a bit complicated. I’m sure there are other ways to do this. And plenty of improvements are possible to this method. Please comment below. Thanks, Bill

Firebox Cajon with polycarbonate back, red cedar and yellow cedar sides top and bottom.
Firebox Cajon with polycarbonate back, red cedar and yellow cedar sides, top and bottom. Made in Alaska, USA
Firebox Cajon with luaan tapa, red cedar and yellow cedar sides, top and bottom. Tung oil finish. Made in Alaska, USA
Firebox Cajon with luaan tapa, red cedar and yellow cedar sides, top and bottom. Tung oil finish. Made in Alaska, USA

Attaching polycarbonate as a cajon back:

Polycarbonate gives a cajon a lot of resonance. I cut the poly with a ‘ceramics’ blade on a jig saw which is much, much safer than other methods I’ve tried. Then I sand both sides to ‘frost’ the sheet with either a belt sander or an orbital sander. After cleaning the inside edges with alcohol the polycarbonate it is ready for ‘gluing’ with either 2  part epoxy or AquaSeal as the adhesive.  Critical is an epoxy that is not runny. It needs to be a bit thick to fill and adhere.  If you are using epoxy you must work fast to coat both the wood and the polycarbonate before the epoxy starts to set. Any end grain must be sealed with a thin layer of epoxy the day before glue up. Many epoxies are heat soluble, if you make a cajon with an epoxied back don’t let it get to hot. And it’s a good idea to put a few screws in to help ensure it stays put.

2 Replies to “Building Cajons – Attaching the Back Panel Without Clamps”

  1. Nice! I really appreciate this post. I’ve been looking for different methods and ideas lately. Is your soundhole on the bottom/front?

    1. Thanks, I’m happy with this method so I thought I’d share it. Yes, the sound hole in on the bottom. It is ‘offset’ in that the back of the sound hole is lower than the front. This, in theory, sends the sound out the front. Beating Cajon in Japan makes cajons with the sound hole on the bottom but the hole is just a standard circular hole. There is a difference in tone depending on if the cajon is on a hard surface or on a rug. I actually prefer the rug, but perhaps just because I’m used to it. Next post will be show I build them.

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