For shims, I used some washers and a cut up soda can. Rubber is not a good choice. When I align, I align to the lens board rather than the end of the lens. I figure that the lens company could care less how perpendicular the lens axis is to the end of the lens, but they really care about how perpendicular the lens flange is. That means you should really make sure the lens board is parallel to the carrier rather than the end of the lens. I shim the carrier or the lens board (depends on enlarger which is easier....lens board preferred since it moves less often) then align the head to the baseboard.