Couple years back, I installed two "Dead Mule Recoil Reducers" in a 12 gauge for a fellow with about the same problem as you. He liked to hunt ducks and geese, and couldn't get the results he wanted by using a low-powered load.
These fit in a 7/8" hole in the butt, under the butt plate, and are available from Midway USA. IIRC, the cost was about $80 each.