After a cartridge is fired, it expands, that's why they need to be re-sized when you reload them. Without re-sizing, the bullet is a slip fit in the fired case. Being all you want them for is display, the easiest thing to do is to glue them in place. This could be done with J-B Weld or similar epoxy.
Check the cases for any with out of round necks. Use a slightly smaller 1/4" drive socket to make them round again. Test with a bullet before gluing. Clean inside the neck with lacquer thinner, acetone, or similar solvent. An asphaltic sealer is used to make the ammo waterproof and you want to clean the residue of that out so the epoxy will bond.
Apply the epoxy with a toothpick to the inside of the neck. Slide the bullet in up to the cannelure and set aside to dry.