I ordered this from Northern Tool:
https://www.northerntool.com/shop/to...0268_200660268
So far it keeps my battery fully charged while stored, been rained on plenty of times without any problems. Just remember to keep the battery's water topped off and brush away any snow.
Here's a picture of my setup:
I leave my cutoff switch off when the panel is in use. I just hooked the clips from the panel's wire directly to the battery posts.