I used shower pan liner from lowes, its about 30 mil PVC and you can get it by the foot off a roll or in a sheet. Rear is just a flap but the front is a little more tricky to pull off without creases and have look professional. After putting in a pond last summer I think pond liner would be perfect. It comes in 45 mil EPDM rubber which is very stretchy/flexy and UV/chemical/weather resistant unlike PVC. You can get it on ebay or any pond supply store. Either way clean up the frame a bit and hit the frame and gap guards with undercoat and they will blend right in.