When I added more batteries to Mirador I needed a new box that could hold four T-105s. I was planning to build one as you suggest but found a really nice one at the Batteries Plus store where I bought my batteries. The store I use has a large shop in back where they test and charge batteries. They had a lot of different "plastic" boxes that held different size batteries. The one they sold me for $60 is very stout plastic (polyethylene I think) with a lid that fits snuggly over the top. The box holds 4 T-105 six-volt batteries with absolutely no room to spare.

I secured two sets of nylon straps the outside of the box that hold down the lid. I then bolted the box to the platform I built behind the engine.

Waterproof, acid resistant, will retain the batteries in case of a roll over, and cheap.

See the attached photo