I used some leftover adhesive foam from my dash project to seal up the unused openings in the box - if you don’t have that, you could use gaffer tape or duct tape.
You will need to enlarge some holes in the box, seal the unused ones up, mount the vent connector from the old box, and mount the drain “pipe” in the floor of the van, as well. Save the fusible link, mounted on the side of the box - you will need it later: I cut the existing box up, reusing part of the box for the vent and the drain: I am comfortable with that - you may not be. My method does not secure the battery in this manner, but relies on multiple screws, and the battery box itself. The existing battery is strapped in place with bolts that go through the battery box and floor of the van. Here is the old box, without the battery: Note which wire is connected to the chassis - this is the negative wire. You may wish to label the wires in a manner that is clear to you.
This required removing the existing battery box, installing a new box (after modifying the box with parts from the old box, as well as latches and seals), installing a smaller box for the battery disconnect switch, and while I was at it, installing a new battery meter. More power in almost the same space - how cool is that? The 12 volt battery we had (Trojan 30 XHS) provided 130 amp/hours - the 6 volt (Trojan T-105) provides 225 amp/hours. Our reliable Trojan “house” battery was nearing the end of it’s usable camping life (no longer as many days between charges), so I made the switch to a dual 6 volt system.