I've never really understood why fin manufacturers back in the 1980s discontinued using natural rubber, which always keeps its shape, to make foot pockets, substituting "thermoplastic elastomer", which sags and warps without support. Actually, I do understand why. TPE may be easier, and therefore cheaper, to manufacture than natural rubber. It may also be easier to bond TPE to plastic blades for all I know. Any other benefits to the user escape me, which is why I stick to all-rubber fins, which don't come with - or require - plastic "formers" to stay in shape.
Perhaps you could find something in turn to substitute for the plastic formers you've mislaid. Consider stuffing the foot pockets with plastic bags or crumpled-up newspapers. These fillings may end up doing a better job than the plastic formers, which seemed to have quite sharp edges the last time I looked in my local dive store and therefore did the material inside the foot pockets no favours.
