As the OP already mentioned and if I also wanted to do this, I would just go with the
Jtek Shiftmate converter thingamajig, around $35+. Throw either a 9 or 10sp Shimano cassette on and the Shiftmate #1 can be setup for either. No farting around with spacers or expensive converter cassettes.
Shiftmate Compatibility Charts and Choices - Jtek Engineering