: And for future glassing what would be the best thing to place under the seams?

Good old polyethylene plastic sheeting (weatherproofing, paint tarps, trash bags, etc). I like to use 6 mil to cover my workbench when epoxying. The epoxy pops right off after it cures.

Clear packing tape is an easy release agent when gluing up coaming and hatches when you are not ready to attach them to the deck but want to cold form them to the deck. I use the standard weight 3M Scotch brand because I also use it for stretch tape to help clamp on decks. It does not leave the PSA (pressure sensitive adhesive) stuck on previously epoxied surfaces when you peel it off (like cheapo brands do). And the standard weight stretches more than the heavy weight for better clamping pressure.