I've been fiddling with my latest pair of Voit UDTs for about 4 months now, making changes in the length, flex, foot pocket padding, and heel strap padding in small increments.
I finally got the foot pocket padding right, so I committed the combination of two full length 3mm wetsuit pads, one 3/4 length wetsuit pad, and one plastic top stiffener to a more permanent state.
As you can see, I sandwiched a thin sheet of water-jug-plastic between layers of neoprene using a spot bonding technique. No easily obtainable adhesive (that I know of) will stick to that plastic material, so I allow the neoprene layers to bond to themselves through holes punched in the plastic and a narrow rim around the perimeter. This firmly holds the plastic between the neoprene layers. (The bonding glue was Barge contact cement.)
The plastic was pressed onto one side of the neoprene after the contact cement had dried to the point where it was ready to be fit onto the other layer of neoprene. (About 20 minutes.)
After the neoprene/plastic sandwich was complete, I tucked the "tongue" back into the foot packet, yielding this result...
I began using the plastic inserts on the inside/top of foot pockets when Voit Duckfeet started putting softer rubber in the foot pocket area of their fins back in the 80's. UDT's have very good foot pocket rubber, but over the years I've learned that I like a really stiff foot pocket (which is padded with neoprene) and a more flexible blade.
I've used as many as three layers of plastic on some poorly made fins, but one layer is sufficient on UDT's.