Also keep in mind that most epoxies have a hard time with UV exposure - they turn yellow/hazy and eventually get brittle. You will need to put a coat of paint on the epoxy or maybe some sealant to help block the sun if you use epoxy to fill any the difference in curvature.

Secondly, if you coat the plastic with some sort of a mold release, you will be able to remove the plastic port once the epoxy has cured. PVA mold release would work well - but that's an extra expense. You can also use wax paper, any kind of polypropoline plastic (like cheap drop sheets), or even grease as a barrier between the port and the epoxy. I haven't tried this but I believe if you put a skim coat of silicon sealant on the plastic, it will also provide a good barrier. This way you get a good solid mount but can still remove the port easily should it ever be damaged or deteriorate.