Goirish:

1) The shim behind the mount proposed by MinnFlyer is a great solution.

2) If you glue some wood inside the cowl, you will have enough meat to sand back, as RCKen suggests. Use epoxy instead of CA, in order to stand the fuel.

3) You could also add a metal washer as a spacer between the drive washer and the spinner back plate.

Regards!