Probably 7+ years ago there were lots
of posts on the subject. Many people used/modified various metal truck boxes to fit their needs.
I would take a measurement across your A-Frame, to determine how much length you need, to span a box across the frame members. Then find a box that will fit your needs.
One thing to note, if you have open cell batteries that need to be maintained, your battery box will need ventilation. No matter what, I would also drill a few weep holes in the bottom so any condensation build up has a way to escape.
Amazon is a great place to look at options.