The powder in the loaded muzzle loader should be protected from moisture and patch lubrication by placing a layer
or two of wax paper, a dry wool wad or waxed card over the powder, or all of the above.
Or else the lube can possibly migrate into the powder over time and make it too weak to set off.
Placing a piece of tape, part of a balloon similar to a rubber "weatherguard" condom like device on the muzzle will
further protect the bore and powder from contamination by moisture, humidity, condensation and dust.
There's different kinds of nipples and hole sizes but it wouldn't hurt to use a cap guard of some sort to keep moisture, dust and debris out of the nipple.
The brass cap guard was shown, and a similar replacement would be the screw on cover for a tire valve stem with the hammer left down on it.
Leather is also an option to place under the hammer.
It may also be a good idea to protect caps from humidity, dust and oxidation.
I'm not saying that they won't work if left out in the open, but if using one for home defense you'd want to make sure it will work.
Keeping a capper in a sealed Zip Loc bag or a plastic bag that's folded over may be better than not storing it in plastic at all.
I bought a used cap & ball revolver that was left loaded for at least a year or two that had Bore Butter over the tight fitting balls and the fired loads were anemic.
Either the lube or moisture had migrated into the black powder past the balls or through the nipples.
When fired, the balls could have bounced off a person.
So protect the loaded powder by every reasonable means and remove as much oil as possible from the entire bore before loading the powder into the breech.
Protect your capper with plastic but you'll need to have it ready for action.
Depending on your living situation, the caps should probably be stored in a "safe" place away from the gun, children and untrained or untrusted people.