Because I have had various diameter wheels it has always been a problem to get the spare to sit correctly on the frame. Many seem to use pipe lagging on the frame to compensate but it's ugly.
My solution was to make the mounting point for the bolt holding the wheel adjustable. I achieved this by taking a piece of flat bar about 4 or 5 inches long with a tapped hole on one end and a large slot in the remainder. The bracket is attached to the car using a bolt through the slot allowing the fixing point (tapped hole) to move up and down. Not easy to describe but hope you can understand. I often thought that there is a market for such a thing.