Well, if color temperature isn't critical, it's just a piece of translucent, white acrylic plastic mounted above a light source - preferrably a broad, uniform source - like an array of fluorescent tubes. If you use incandescent bulbs, or more than a few fluorescent tubes, it'd probably be wise to incorporate a fan into the box holding the tubes.
I'm sorry that this isn't too useful, particularly since you probably want a reasonably good box. With the selection of fluorescent bulbs available today, you ought to be able to do pretty well, though. I'd buy an inexpensive, undercounter 24" fluorescent fixture or two and mount it (them) upside down in a wooden box, with a rabbet around the top - then put on a layer of 1/4" white acrylic. If that didn't give enough diffusion, maybe a second layer of acrylic. In thinking about it, I'll bet you can find a fluorescent fixture with an adequate diffuser built on - and just mount it upside down.