11t cog will be thinner, so just sort your spacing remove the 12T not the 11T (which sits over the end of the cassette)
or do the decent thing and use more cogs at the back and use it as 11 speed (to get a smoother cog to cog, will be horrible on 9sp, hence why the big cogs have now appeared) or the even simpler thing, heaven forbid, and use multi rings at the front.