If you're using the light table for viewing photographic work, transparencies etc., I would suggest using 5000ēK fluorescent tubes. The top could be opal plexiglass topped by a sheet of 1/4" plate glass to protect it from scratches, dirt etc. I've built a number of these over the years and they work great. I'm sure others here will suggest different methods and materials , but this is one possibility at least. Hope this is of some help.