I have a nice set of Zeiss lenses for the no longer existent Rollei 35mm SLR system. I want to use these on my newly acquired Sony A7 with an adapter. The problem I have are the terrible friction fit rear lens cap supplied with these lenses which fall off so easily. Has anyone found a solution to this problem?

Tamron actually makes a nice rear cap for their rollei adapters but they don't seem available and the lenses aren't popular enough for the Asian block to manufacturer. The other solution is to buy an adapter for each lens but that seems very wasteful. I've tried all the rear caps I have and the only ones that come close to working are some OM and the Sony E (Minolta, Nikon of course being backwards, Leica of course, Contax G, m4/3, don't work).