Once you have the general aim point established, you know the forward and rear face of the mast relative to the station of interest.
Cut two pieces of sheet metal... so that each will wrap about half way around the mast. Place one on the forward face of the mast, the other on the rear. Hold them in place temporarily with tape until the antenna clamp holds them. The shim on the forward face needs to be lower than the rear so that only the bottom of the clamp contacts the shim, the front upper clamp 'teeth' need to contact the mast directly, not the shim. At the rear, the shim needs to contact only the upper 'teeth' of the antenna clamp.
If needed, add more shim stock.
An aluminum soda or beer can can supply the shim stock. Your wife's best sewing scissors can cut the aluminum. Or maybe not
