You can put a piece of waxed paper, or plastic wrap under the patch and ball at the muzzle, cut the vapor barrier and patch at the same time, and load them down onto the powder charge together to protect the powder from moisture infiltration from either rain or condensation in the bore.
Some people put tape over the muzzle to keep condensation out- but its still not wise to take a cold barrel into a warm home, or camp cabin. Leave the gun outside.
If you use a flintlock, fold a cleaning patch after soaking it with alcohol, and after dumping priming charge from the pan, put 1/2 of the patch in the pan, and the other up against the barrel covering the Touch Hole. Close the frizzen on the patch. The alcohol will evaporate, and in doing so remove any moisture that may have gotten into the powder charge in the barrel during the day's hunt and handling. The alcohol in the portion of the patch under the frizzen will travel up the fabric, thru osmosis, keeping alcohol as a barrier against that TH overnight.
If you are shooting a percussion gun, put a piece of plastic wrap over the nipple before you put the percussion cap on it. Once the cap is in place, lower the hammer, and cut, or break off any excess plastic around the cap. When the gun fires, the hot flame will instantly burn right thru the plastic, but the plastic will act as your vapor barrier as long as its in place. In theory, a gun loaded with vapor barriers at both ends of the bore should be able to be stored indefinitely, and still fire on demand.
There are other choices or options to use in sealing the nipple from moisture, but this is the cheapest and most efficient one available to day.