What worries me is water running down the stock and under the plastic.
Are you planning on laying your gun in a creek? 
Seriously I think you guys are going a little overboard IMO. I've hunted in a lot of rain with a muzzle loader and never had a miss/hang fire. Just make an effort not to get it soaking wet and that in itself will go a long ways. If it does get soaked, at the end of the day fire it, then fire a couple caps through it and make sure it's dry.
Even with good rifle care, it can and does happen. I just got back from the Olympic Penisula, the rain was incredible. We taped our muzzle and had rubber to cover the nipple before capping, and almost never had them capped. My buddy's rifle after day two would not fire, pushed the load out and the powder had turned to jelly. We are certain the water came in through the breech plug. Solved that problem with Teflon plumbers tape.
Another cover for a barrel is the cut off finger if a black Latex glove.
Good luck with the rest of elk season.
Scud
