Go down to your nearest hardware/industrial supplier (if you have a Blackwoods or a Bakers nearby they are perfect) and you can get brass shim in sheets, rolls or in cans (the size of beer cans) containing sheets of varying thickness.

Or you could just cut up aluminium cans with some old scissors.

A set of springs on the mounting screws would probably help even out the pressures and should be available at a hardware place.