I had a brand new marine battery for my little electric trolling motor. I also had a new PLANO tool box on hand.
The box fit perfectly into the brackets where the old battery box was fastened. I screwed the box to the bracket with six healthy screws. The batteries sit in the box at just the right height so that it requires a bit of a squeeze to shut the lid.
There is room for a battery of equal size to the bigger one. It fits a bit tight against the propane tank, but it works okay.
Tomorrow, I'll go to the local CO-OP and get what I need to hook the batteries parallel. More luck than skill or brains involved here!!