I built a wooden frame with plywood bottom, 26" x 50" and packed it with as many 2-tube 48" fluor fixtures as wood fit (10 "full spectrum" 5000k tubes). On the inside of the frame, I glued aluminum foil as a reflector. For the top, I bought a sheet of 1/8" white plexi, and covered this with a sheet of 1/4 plate glass. The plexi and glass fit in a rabbit in the frame--leave some room for expansion as it warms up. The plexi cuts down quite a bit of light, so you need all the light you can produce, but it does give a very even light. The glass allows me to work on the table, fairly safely. (Never broken it in 3 years).
This is my 3rd lightbox, and the best so far.