If it has a curved metal butt plate, about all you can do is cut the butt flat and install a 1" recoil pad or shape a piece of similar wood and glue/screw it on and cover with a slip on leather or rubber boot.There are some pretty nice commercial hand made leather boots that would cover it all up

If God wanted you to walk and carry things on your back, He would not have invented stirrups and pack saddles