I couldn't find a leather maker who had anything to fit mine with 11" barrels, they all said something could be custom made, but nothing off the shelf. That, of course, presents a problem unless you're local.
I doubt a holster for a single shot .410 is going to handle a double 12 gauge...
Should you come up with a maker who has one that fits, I'd like to know. As it is, I'm talking to a gentleman in the town next to me who makes nice cowboy action leather about a custom job. It would mean a lot of traveling back and forth for fitting, but I'm not sure of any other way around it.