How about a good silicone glue? (Like aquarium sealer)

Tar like substance might do better as it would give a flexible bond.

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The compression of a rubbery substance is a wothwhile direction. We
have one berm pond with a toilet flange as the exit. It is rather
like a burger: The flange is the outer layer (bread/bun). Then two
layers of EDPM, coated with roofing sealer on either side and between
(lettuce and tomatoes?). Then the plastic (burger in the center). It
repeats on the other side with two layers of EDPM and a flat toilet
ring. The flange and the ring are held together by stainless steel
bolts and washers. The bolts compress the edpm and tar, resulting in
a rather good seal. We have used that technique on the berm pond
drains as well. They are holding well with edpm liners as well.


Jim