Doesn't everyone have a flintlock fowler (or at least a caplock double)?Doesn't everyone have a stash of left over 57's?
I've used the same approach as Mr. Ross: trim plastic shells to length, build a load column (powder/wads/shot/overpowder wad) and roll-crimp the case mouth. These work very well in some of the older shotguns I've played with over the years, particularly those with short chambers. I still kick myself for trading off a Lefaucheaux underlever with 2" chambers, I've never found another gun that worked so well for me when shooting doves.