How To Build a Cajón
The basic box is constructed from 1/2" mahogany plywood for sides, back, top, and bottom and 1/4" mahogany plywood for front (tapa). Size is 13" x 19" with sides sitting on bottom board. Front and back extend to widest and tallest dimensions. Cut a 4" hole in the back. After pieces are cut and fit, apply carpenter's glue and clamp sides, back, top, and bottom together. Round all corners except bottom by hand or with a router.
A set of guitar keys make tensioning the A and D guitar strings easy. Mount the keys so they almost touch the inside back with the first key about 8" from the top.
Glue 1" x 1" hardwood rail pieces flush with the front. Drill holes close to top and sides for guitar strings. Start 3" from side and space 1" apart at top. Drill side holes to correspond to key spacing.

Left is the back with keys in place.
Above is the top.
Right are the guitar strings. Use the keys to tension and "tune" the strings. The strings make direct contact with the front. A little experimentation will give you the sound you're looking for. Below left shows the strings on the keys. Note they are also in contact with the sides adding to the snare effect.
Below is the finished Cajón with brass screws spaced 2". The top is left without screws.
Finish the Cajón with wood stain. By loosing the brass screws a bit, the string tension can be changed to give a different snare sound. You may also want to add a small tambourine that can occasionally be played with one hand while the other is drumming. Cost for all materials is less than $50 U.S.
For additional information try this site: http://en.wikipedia.org/wiki/Caj%C3%B3n