It looks to me like the heater flaps are constructed with a metal base into
which a plastic rectangle was pressed, the foam seal being held between the
plastic and the metal (don't know if the foam was stuck to the plastic or
just held in place by it).  Should I

(a) leave the plastic there, and stick my foam to the plastic?

(b) remove the plastic and stick my foam to the metal?

(c) remove the plastic, put my foam in, stick the plastic back on?

mike