Just food for thought---- Have you tried the gun with the riser in question on clays? The reason I ask is because I own a Pigeon grade gun with a Monte Carlo stock (factory) that is very similar to your picture, and there is no way I could shoot clays with it. The "riser" makes it shoot way too high for me. Perhaps a "standard" stock would suit you better.
Jerry Andrews may have a good used regular frame stock that would work.