Going to go with steel for the adapter. To get a good check on my measurements I've been turning each face separately out of aluminum to make sure my measurements are really accurate. I've got the fit onto the stock shaft perfect, so next I have to cut that out of steel rod. Then do aluminum turns to fit into the CBR rotor before transferring that angle onto the steel rod. For the final interference fit I saw that people will use a little valve grinding compound and just spin the parts against each other.
Working a lot on the seat. Almost done with fiberglass, then on to bondo (which is heavy as hell, that's why I want to get it really close with 'glass).