Hello hlw49,
Heat molded seat assemblies can be a real pain to work with.
For extreme cases, I usually rough up the plastic or sand the metal and then use expanding foam in a can with a putty knife. After that I use plenty of gorilla tape to hold it together until the foam cures. Once cured, the tape is removed.
It takes a little practice to realize how the expanding foam works and holds. That stuff does not give up once is sticks the seat materials together.