I used plexiglass, the thickest they sell at Lowe's (I think it's 1/4 inch). I think mine is around 600 GPH. But from what I read when I built my sump, the flow rate shouldn't matter when it comes to the positioning of the baffles. I use an Eheim 1200 for the return pump, which is 1200 GPH. But my overflow box is rated at 600.