Creeper, I just wish Browning produced an extended mag so I could have 5 shots at my disposal.
Just out of curiosity, I searched for about 15 minutes... nothing. However, I'd be willing to bet that a shotgun specialty company like Briley or Sure Cycle would entertain the idea of making you one. Even a sleeved down 12 ga would be feasible. Sure, it might set you back a few bucks, but it'd sure be neat.
If I still had a lathe, I'd be able to make you one in a few hours... have any friends with a lathe?