You'd be much better off using 1/4" acrylic and bonding it to the sump wall with a product called weld-on #40. That's kind of hard to find, so maybe the next best thing would be weld-on #16. I don't use weld-on #16 for joints that have to hold water pressure, but since a baffle is internal only it should be okay. If you can get a PERFECT fit between the edge of the baffle and the sump wall, you can use weld-on #4 which is a solvent that welds the two pieces of acrylic together. A comromise might be cutting the baffle so it's the best you can do, hopefully pretty good contact with the sump walls, and using some #4 to hold it in place, then running a bead of #16 (#40 is better) along the joint to stregthen it. I disagree with Ace's idea of bowing the acrylic to make it fit, unless you're talking about a very tiny amount of bow. If is too much (really anything you could see would be too much IMO) it will just stress the joint.