They all barely fit in there.
I'm trying to decide how to do the tie-downs. I had been using two threaded rods on either side of the Group 31 battery. A piece of "unistrut" spanned the battery to either rod, and was tightened down with a nut and washer.
I think for the four battery setup, I can still use threaded rods. One in the middle, between all the batteries, and one on either end near the headlights.
At the
Milwaukee Makerspace (my car repair facility and hang-out for the weekend...) there was some nice, beefy, flat bar stock on the misc metals rack. Those pieces were about 17 or 18" long each - just about perfect for this.
Also, I rotated the batteries so that the closest connectors are +,-,+,-......
That makes it so that a hold down bar can go straight across all the batteries, without having to compete with a crossing battery cable.
Then that's the other big thing to do, make batteries cables for this.
Oh, yeah. I also have no real working battery charger right now. Maybe somebody could help me build a variable voltage open source battery charger.
Hint, Hint, wink....