When the plastic comes out of the vacuum-form machine, there will still be a lot of leftover plastic that needs to be sliced off (namely, with a band saw). Also, since I want a nice, clean hexagonal shape for my bracers, I need to cut off all excess plastic as well as the foam that didn't fully meet up. I used the wooden re-saw guide to hold my pieces up at a 90° to get an accurate slice.
Now, with all but the bottom of the foam pieces shielded with plastic, I can simply epoxy a fresh sheet of plastic to the bottoms. After that, I cut the bracers in half again to reveal the plywood inside. As you can see, the blue foam is now 100% protected by stronger materials, while keeping its own shape intact.
This is the Spindle Sander (which I used earlier on when I drilled the holes into the foam, but didn't photograph). Right now I just needed to sand away the new sheet of plastic until it matches the shape of the rest of the bracer.