Thanks everyone! I hadn't considered running beside the vent pipe - that's a great idea!
BTW - As I mentioned, I had to build a custom battery box since none of the commercially available boxes would fit. While a metal box was my first choice, I don't have a sheer or a brake large enough to work with material that size - and due to the tight fit I didn't want to pay a shop and then keep going back for mods when it didn't quite fit. So I made the box out of Baltic ply, saturated it with expoxy, covered it in fiberglass, and painted it with exterior paint. Should hold up fine.
Here is the box with the lid off -
And lid on -
