I recently made one out of a 40g breeder with thin acrylic sheeting.....not a good idea. I filled it with water and turned the pump on. My drainage and return worked great, but my acrylic couldn't handle the water pressure, bent, and eventually came loose. Definitely a rookie mistake that could have been avoided with a little more research or just asking a question on the forum... [smilie=angry.gif] . Now I have to drain my sump and redo it with glass...
My advice is if in doubt....go thicker!