If you go to an automotive supply house, they will sell you a sheet of "artificial gasket material". The stuff is like heavy paper and comes in several thicknesses. It can be cut with scissors or an x-acto knife. Trace & cut out the shape you want and then either laminate it or take a Dremel to grind down one side to angle the shim. If you want it fancy, you can paint it with clear nail polish. It takes me about 15 minutes to make a shim for a Beretta or Benelli auto.