i had the same problem when i got my rims, they had caps that said 'excel' on them and my standard subaru ones wouldn't clip in there properly, so i got some o rings and stretched them over the subi caps and that was enough for the centrecaps to sit in the rims really nicely, about 50,000ks later and they haven't budged

your car never leaves your garage anyway T, just blutack them on LOL