I built this in two days, mostly out of scrap from my shop. The top, bottom, and sides are 3/4" baltic birch left over from a dresser project a few years back, and the back is some 1/4" lauan left over from some bookcase backing. The main soundboard is 3mm birch from the local Woodcraft. The corners of the box are just rabbet joints with no reinforcements, and it seems to hold my weight just fine. The box is roughly 12x12x17, mostly just a function of the plywood scraps I had to work with. To get the red, I used about 1/3 bottle of transtint liquid red in a quart can of universal sealcoat (dewaxed shellac) -- the main box got two coats of that, then everything got 2 or 3 coats of a water-based hardwood floor finish. I stole two major design ideas from a couple of guys on YouTube:
(1) Snare attachment: there's a dowel running through the middle that holds two halves of a replacement snare, attached to two knobs on the sides. The left knob adjusts the position of the snare, either against the soundboard or against a dowel farther back to keep it from ringing out, and the right knob just tightens it into place.
http://dan.aleph0.com/cajon-sample-no-snare.mp3
http://dan.aleph0.com/cajon-sample-snare.mp3
I'm surprised at the volume and variety of sound I can get out of this little box. The snare doesn't make as much difference as I'd like, but I'm working on that -- I suspect that the dowel supporting the snares is too far from the top, so that the length of snare vibrating against the top is too short.