A successful ‘Flopper Stopper’ principally has to do two things. (1) Sink easily. (2) Resist an upward force. One simple method is to have a weighted horizontal door. The ‘door’ opens on the way down (sinks) and ‘closes’ on the up lift from the halyards, a bit like a one-way valve. I have plans, in my head, for a stainless steel flopper. Sort of like a flat door in a four-sided door jamb, the whole thing being held by a halyard at each corner. That was not going to happen off the coast of Ecuador today.