Are you making a POP mold? I've used only double-sided permanent Scotch tape to stick my plastic masters down. It works best for flat-sided baits but also works for round ones. Cover the entire area where the master will be with the tape. Wash your plastic masters with dish soap to remove any oils, dry, and then firmly stick them down on the tape. Once in a while I'll have one of the masters float up on one end but it works 99% of the time. It will not ruin the master but it will have sticky residue on it. Hopefully when you get one good cavity you really don't need the master any more.
Excuse my rambling on but if I have only one master I sometimes make the mold the old-fashioned push it down in the POP way. If your POP is thin enough this will work fine. You must stand over it and keep pushing the master down until it stops floating. That way you will have a master with a little flat side that can be more reliably stuck down on tape to make production molds. Good luck.