Welcome to the Forum.
The other suggestions are good and will help you have a successful hunt.
The Cows knee is kinda like a little tent that fits over the lock and the top of the barrel next to the lock.
Just about any waterproof material will do, even a plastic bag but it is better if it is made out of something a bit heavier.
These are often tied to the trigger guard with a leather thong but if you don't have one a piece of heavy string or cord will do. The idea here is that when you flip it off to take a shot it won't be lost.
Another thing that can be done is to coat the area where the top of the frizzen meets the barrel with some grease to stop the rainwater from getting down into that area.
This can be kinda messy and of course, you will have to reseal it after re-freshening the prime.
The thing to keep in mind if you do this is you don't want any grease to get on the face of the frizzen or to get down inside and block the vent hole.
A wet frizzen face will spark with a good flint so make sure your flint is sharp and adjusted right.
Good luck to you on your hunt.