The problem was that the "knobs" that held the heel straps on were molded out of plastic embedded into the fin instead of the rubber the rest of it is made of. This plastic apparently doesn't hold up so well.

My fix was to drill out the old plastic and create an insert that goes all the way through the fin and a cap that screws on, capturing the insert. Alas, I don't have dimentions or anything any more. I did this as a "one off" quite some time ago. It saved me from having to buy new fins...

Broken

Fixed, top view

Fixed, side view

Fixed, inside view

The parts created for repair