I drill the cap first. Being a woodworker I have "brad point" bits that cut a clean hole.
It could be done with a standard bit tho, then carefully cut off the sides of the cap with an X-acto.
Scissors will also cut the soft bottle caps.
Or just take a piece of a bleach bottle and make a spacer from that.
They are about a half a ml. thick.
Of course this is me being
thrifty and cheap, but may get you by, until you find something like the ones FT sells.