I currently have someone making me up an 8mm foresight raising block - mine will be to fit on a 19 series dovetail and take an 18 series foresight.
Not expensive.
I'm sure he could do a straight 18 series block too.
In fact, Barry Nesom made up one of those for me a few years back.
Anyway,
PM me and I'll pass on his email address.
Ken.