Update: I may be getting carried away... Lol.
Not sure if I'm just crazy, or drunk off the good results I got from casting a mold of the boot treads...but I'm going to attempt more rubber molding. My new idea is to cut out the circular opening around the arm/shoulder joint. I'm going to try and cut these out of the chest and back pieces, glue them together, smooth and shape them, make a ledge for connections and magnets, and make a silicone mold.
After that, I'll cast the mold in a flexible rubber. I'm thinking of mounting the rubber ring permanently to the back piece, while attaching some magnets to the front of the ring. That way, when I attach the chest piece, the rubber ring will snap into place with magnets, forming a solid circle around my shoulder and helping a little to hold the chest in place...
I have 3 extra prints for the chest piece (scaling that was a pain), so I should have plenty to practice on before attempting this on the properly scaled chest piece. I've included a pic of my first attempt on a spare chest piece. I'm using a hot-wire foam cutter, which seems to be working very well. Because the wire is so thin, you have to move slow and make several passes to get a clean enough separation of the pieces. After just one pass, the melted plastic on the cut pieces kinda half-weld back together.
The reason I had for trying this was to increase the comfort and flexibility around the arms and shoulders. When all is said and done, I want to be able to remove my own helmet, and reach the rifle I plan to attach magnetically to the back piece.
We'll see how the test piece goes... Fingers crossed.