Building a light box is dead easy. All you need is a bit of MDF
(laminated on one side if you want it neat), some natural light (5000k) fluorescent bulbs
and fixtures, some white paint and some frosted perspex (tell the supplier it is for a light box, they will give you the right stuff).
Cut your MDF to be able to build a box that is about 8 inches high, 6 inches longer than your bulbs and is (8" x no of bulbs + 8") wide. Do not cut a lid, this will be the perspex.
Build the box and paint it gloss white on the inside. Next screw your light fittings inside, threading a power cable through a hole in the side. Next attach the perspex lid (as light tight as possible and also removable to facilitate bulb changing, and you have a light box.
The benefit of doing this rather than buying one is for large sizes (A2 and up) you save a lot of money, and the result is just the same.