FYI I recently picked one of these up a few months back and finally got around to rigging up a lightbox.
The glass dimensions are 13x17" for the glass itself with 12x16" being unblocked by the mounting flange of the table itself (the glass is inset to the table). So I picked up an 11x15" Testrite lightbox from Freestyle and suspended it with framing wire supported by hooks screwed into the bottom of the desk. Took around 30 mins or so to do. Very easy - just pilot-drill (which is good practice anyways). It's basic home-engineering to get a lightbox in there. The glass itself is already frosted, but having tried the lightbox without the plexi the bulb was still plenty visible. I ended up keeping the plexi diffuser on it while under the glass.