I managed to pick up a Soviet turret finder for my FED2 at a sensible price. Unfortunately, it is so loose it just falls straight out of the accessory shoe! Should there be some sort of spring clip to hold it in place?
I have the same problem with a Fed S. I have tried a piece of Felt (feutrine in French): a sort of soft material in auto sticking version. It exists in several colours, but I suppose it's not the problem!