I suggest a simple solution that was used 300 years ago until after the Civil War...make paper cartridges! They are quick to open, stay closed until needed, and are reasonably water resistant- I once dropped several into a creek by accident and when I fished them out the powder was still dry.
Find a dowel a little under bore size, around 6 inches long. Cut regular copy paper into 4 sections (fold in half, and in half again, cut on folds) wrap the paper around your dowel (former) and then pull the former back slightly more than its diameter. Starting at the overlap, press the end in flat. Then one side, then the other.
Now, to ensure the paper stays closed- pull the former slightly back, and press the folded end in. Then crimp the edges down, making something like a shotgun shell crimp.
Remove former. Repeat and make several cartridge forms. Measure powder into one. press the paper tail flat, fold each side into the middle. I like to put a small fold at the end of the tail to double insure that no powder can escape.
With a smoothbore, you can of course put a ball in the cartridge first.
To use, just bite off the tail, and dump the powder into the barrel. With my Bess, I can then just drop the ball and paper down to follow the powder. Rifles can just toss the paper, or pocket it. You could wax or grease the paper around a ball when using a smoothbore.
Don't bite off the bullet, bite off the tail.