It is not a litterbox, hubby got it from his work, actually. It is a lot sturdier, taller, and thicker than a litterbox. But, I'm sure you can find something similar somewhere.
For depth, I'd guess you want 2" for the ice trays, 2-4 inches of sand on top, and still some room left to tuck in the sides of that foam insulation stuff. For width/length just picture how many chickens will fit in there comfortably (keep in mind who gets along with whom ...maybe you need more than one box to keep everyone happy...)
Good luck, and let me know how it works out for you.