I find these bottle cap stamps a great idea: simply glue ready bought craft foam pieces onto the bottle caps and you’re done.
Because I mostly don’t like the ready bought craft foam pieces (locomotives are e.g. not my cup of tea ;-)) I thought that I’d better cut the craft foam myself or use thin self carved eraser stamps.
But if you have to know precisely where to stamp I think that the miniature marble stamps are better than the bottle cap stamps.
Links:
Bottle Cap Stamps (Create With Kids: Make Your Own Bottle Top Stamps)
via: Make Your Own Bottle Top Stamps
Here at unikatissima:
Eraser Stamps
Miniature Marble Stamps