I am planing a box very similar in concept to this!! I was going to put the pump, rad, res in the box and hace a variable speed fan controler, thermal overload protection, and leak detection.
I have a lazer at work and a very good source for acrylic. I only pay about $15 for a 4'X8' sheet of 1/4" acrylic. Havn't quite got the drawing of the parts down yet though. I have to many projects I am trying to peice together during the small amount of free time I have.
Anyway I would just use standard 1/4" acrylic and use IPS Weld-On acrylic cement #4 or #16. Preferable #16 if you plan on a water tight seal.