I make cartridges for my mini' s and maxi's out of plastic tubing I get from Brownells with end stoppers.
They are reusable and water proof.
Pull off the stopper, pour the powder in,and push the mini through the tube into the muzzle and into the bore. I drill out one of the stoppers and push a cap into it so one has a complete charge in each tube. It could be made to work with a patched ball as well or you could just use a loading block in conjunction with it.MD