I made a light table years ago. I used PVC pipe for the frame, and a piece of white plastic (I dont recall if it was plexi) it was thin enough that I didnt have too hard a time getting it to bend. I think I got the plans for it from Popular Photo(or similar) The only "problem" encountered was that plastic is petro based, and any light shone through it would be slightly orange. It is still a problem when I shoot macro using a white styro cup around my subject. I cut the bottom of the cup to fit over my lens, and set the cup over my subject for even , non reflective illumination.
Rick