If the sump is an old glass tank, then you can use acrylic or glass baffles and silicone them in place. Silicone doesn't hold real well to acrylic, but there isn't much lateral force on the baffle and if you use plenty of Silicone on both sides of the joint, it should be fine.
If you are using an acrylic tank, then again, use acrylic baffles, but use acrylic solvent to bond the two together.
If you are using some other type of plastic tank (like a rubermaid or something, then you can use a thick epoxy with acrylic or other plastic baffles. If you do this, roughen up the plastic where the epoxy will bond to it for a better hold.
Just use common sense and you'll be fine. leaks are not a big problem for the baffles, you just want something that will hold them in place. Thicker "glues" will fill in small gaps where thin solvents should be used for like materials with little or no gap.
For rubbermaid tubs I believe the baffle will come loose due to them bowing outwards when loaded with water. If you have a glass tank, check out the glass option before going right to acrylic. I made some phone calls and found a place near me that sells 3/8" glass for $10 per square foot, cut to my specifications.