I think putting plastic over the nipple before capping, and plastic UNDER the PATCH before loading, is the best way to protect the powder charge. If a leather pad or soft rubber eraser is placed over the nipple after the cap is removed, no moisture should be able to get down into the flash channel.
I don't recommend taking loaded gun out into cold, or back into warm, because whatever moisture was in the air when the gun was loaded is surely going to condense on the metal inside the flash channel, and foul the load.
I also think that putting a patch soaked in alcohol( denatured, not isopropyl) on top of the nipple will suck out any moisture that might be in the flash channel and powder charge from when it was loaded as the alcohol evaporates. Do this either on the way to the hunt, or at night when the gun is being stored with the charge in it, on your porch or someplace outside that is safe, and dryer than the pouring rain around it.
You can also buy small O-rings that will fit around the base of your nipple. Then, putting a pistol cartridge case over the nipple to act as both a safety device, and to keep moisture away from the caps and powder charge will do the job. I think a .380 pistol cartridge will fit the O-ring.