Moth balls in a sock and then tie the end of the sock in a knot so they don’t come out. I have been doing this in our pontoon boat since 2001 and never had a problem and our boat staying in a boat house on the lake. I also put a sock full of them on the floor of the boat when we cover the boat and it helps tremendously.

careful when you use the mothball in a sock deal.
I was helping a neighbor with his boat and was the victim of stuck wide open throttle.
the sock had got stuck under the throttle arm.
luckily the kill switch worked and we happened to be in open water.
bottom line , secure the sock in the cowling