Mike the cheap feeler gauge sets are usually brass in the thinner thicknesses so they will be easy enough to form to the barrel hook. Shim stock will work fine but it is not available in every auto supply or hardware store so it may be hit and miss finding it and when you do choosing the right thickness may be difficult so you may have to buy several thicknesses to get the thickness you desire. As far as what you will be shimming you will want to shim the inside of the barrel hook.