To answer your last question first, using 2F black powder, the grains are too big to fall through the hole. I doubt 3F would leak out either. You can get your cases or gas seals (the gas seal is just a shorter version of the case) from Dixie. You can also buy cast bullets or a bullet mold there too. These are really easy to work with, although most of my experience is with the Smith carbine. I guess the full case would be the easiest. You just put in black powder and leave enough room to seat the bullet in the case. It may not be a tight fit, so I use cigarette paper around the base of the bullet to create a tight friction fit. Then just load it, put on a musket cap and shoot it.
If you use the shorter gas seals, you can use a glue stick and a cigarette paper to make the cartridge. Run the glue stick around the outside of the top of the gas seal, wrap a paper around it, creating a tube. The paper only needs to be about an inch wide. Put in about 20 - 30 grains of powder, put some glue around the bottom edge of your bullet and seat it in the paper cartridge. These are a little more fragile to store and carry, but once you chamber one, if the paper breaks it's no big deal.
After you fire them, wash your cases or gas seals with soap and water.
I also have a Burnside, which uses the gas seals and I make paper cartridges for them. It's pretty easy once you practice a little. Making rounds for breech loading carbines is quite a bit easier than metallic cartridge reloading. There is no re-sizing of cases or priming, and no tools needed.