If you mean a side lock, take some handi wrap and put that over the muzzle. Then take a tire valve stem cover and cock the hammer back, put that over the nipple and percussion cap, and lower the hammer back down on it. When you want to shoot, cock the rifle and flip the cap off.

You can even thread some fishing line through the cap and tie it off to the trigger guard so you do not even loose it. That should keep you going just fine in wet weather.