I have just mounted an onboard charger on the transom wall and made a weather cover for it.
Heck this one is suppose to be waterproof and has a three year warranty. https://www.minnkotamotors.com/Batte...sion-Chargers/
I had an earlier model Minn Kota 6amp single bank on a jon but i only got about three years out of it, maybe this one is better.