A local plastics fabricator cut a piece of 3/16" white translucent plexi #2447 down to size and polished the edges. The 31.5″ x 43.125″ piece cost about $65. I chose two 24' lengths of rope lights over fluorescents for light distribution and safety, since they don't warm up and there's no danger of bulb breakage. Using white foamcore, I built a bottom for the table to concentrate light and give the rope light somewhere to sit. The foamcore is attached to the table with white duct tape (because I'm really fancy like that).
I put a few of the clips that came with the rope lights around the perimeter of the table (pre-drilling holes with a 1/16" bit) and arranged the lights in a random pattern with an emphasis on evenness. The electrical cord stashes easily under the table, and when we are using it I attach an extension cord. For train play we just stick the original tabletop over the plexi.
The whole thing cost under $100. It looks great, doesn't eat up any space and the kids absolutely love it. These days it's dark by 4:30, the light table is a big favorite post-afternoon naps.
(Images: Roni Shapira Ben-Yoseph)