They make this super hard tape now you can buy at Lowes back in the plumbing section. cures with air or something. I would wrap that a couple times around near the end, and then put the clamp on the other side of that stuff. Maybe sand the plastic a little to make it rough, then use acetone to clean it, then wrap the tape, let it cure, then slip the rubber over that. For using the pvc adapter size issue, you could try ABS grind to slurry it in, or I bought 1/8 plastic abs weld rod. You could tape an end on, the wrap it using a heat gun to make it plyable. Then, abs cement to fill the gaps, then abs again for the finish fit. If tight, you could sand it.