mcmaster-carr stainless shim stock, in varying thicknesses. Looks like they sell a sample pack to experiment with.
I bought a sheet of it a long time ago and have cut varying things out of it over the years (Initially to replace a seawitch flap) - works great. In a ramp flap application will last even longer (since there's no repeated bending like on the seawitch flap - which is still ok after about 12 years.)