What works for me with 2.5 inch shells is a dram and a half of American Pioneer fffg equivalent powder, then an overpowder card, then a 1/8 inch thick cork wad punched from bulletin board cork, then the same 2.5cc dipper filled with shot, then an overshot card, then some watered down elmers white glue. You could use a 2.2 or smaller dipper to trim down your load colum short enough to fit in a 2 inch shell. One dram is about 27 grains, and the standard 2.5 inch .410 load is 1 1/2 drams and 1/2 ounce of shot. As long as you use the same volume of blackpowder substitute and shot you should have an OK load.
Some of these new blackpowder substitutes like American Pioneer fill the niche of the old semismokeless powders. They allow me to easily reload some shotgun shells for my 410 at home with just some dippers and priming tools, they are low pressure and pretty forgiving compared to smokeless. You do need to clean your gun more often, but that's just the way it goes I guess. I even made some loads of this type from cutdown plastic shells. They don't look very pretty, but plastic emptys are much cheaper than brass ones, and much simpler than making them form other shells.
Actually, one thing you might consider if you don't have any luck making .410 shells from .303 is to buy a box of Magtec .410 shells and cut them down to the proper length. They cost about $11 per 25, and they look pretty darn good too.
