I've used 2 things to accomplish this with limited success on both. First is clear nail polish. Yep, it works. Plain and simple: better than nothing. 2nd was Uncle Mikes red cap covers. Not better than nail polish, but they work too.
Biggest problem in not getting a muzzleloader to go off is drawing in moisture to the powder charge. This happens because the barrel is colder than the dew point of the area it is stored. The way to avoid this is to not bring the gun into the tent at night or not put it into a warm pickup overnight if the barrel has been cold throughout the day. The dew point inside your warm truck may be at 65 degrees, and a 40 degree barrel temp will guarantee it won't go off in the morning.