Well I have a Remington 870 AOW shotgun and I want to carry it in a scabbard/pouch that will be mounted on the back of my vest (Improved Interceptor OTV).
I hate to make a movie reference, but in the movie Aliens, Corporal Hicks has that shotgun in a scabbard of some sort, on his back, the one that he "keeps handy" for "close encounters."
Obviously this is pre-Molle era, but the movie people had to use some sort of scabbard that he put the shotgun/prop in.
My Rem 870 is slightly longer than 24 inches in overall length, with a 12 inch barrel. It has a Knoxx pistol grip.
I'm hoping to find some heavy duty material that will go via MOLLE directly to the back of my Improved Interceptor.