You can also try seran wrap to wrap around the nipple and tape both ends. The regs clarified what it means to be "eposed to the elements" this year. According to the pamphlet, this means that the nipple cannot be covered or protected by an "Integral part of the weapon proper." So as long as whatever is protecting the nipple isn't a part of your gun it should be legal.
Also, I would recommend shooting your rifle after a day of hunting in the rain/wet snow. I never had a problem either with my weapon not firing until this last year. My dad and I hunted all day in the rain. The next day I took my nipple off to check the powder and it appeared to be dry. I put a little extra in there before putting the nipple back on just to be safe. My dad fired his and it went right off without any delay whatsoever. This also reassured me that everything was likely fine. Well, the next day, my dad and I made beautiful stalk on a nice 5x5 bugling bull getting within about 80 yards in a old logged clearing. I raised up, took aim and PAAAP!! Nothing but the cap went off. Needless to say the whole herd spooked and the bull vanished into the timber.
Thats the last time I'll take that risk after hunting in wet weather.