The battery box is part of the optional firewall forward kit. It includes all the parts necessary to mount (and hold down) the battery on the firewall. Off the top of my head, the part number is PC680 or something like that and you can certainly buy it separately. Just do a search in Van's webstore. There are no drawings that provide dimensions for building it yourself (that I know of), though it probably wouldn't be hard. One other advantage of the kit from Van's, the steel battery box and angles come powdercoated already. Good luck.