Making some progress ... I build a wooden frame so the batteries fit snug in the base. This is a H-shaped oak frame made from 3/4" wood.
It fits in the bottom box, which is an amazingly strong metal plate. It's probably 1/8" thick steel, so it's totally solid even with the weight of the two batteries. Mine are actually a bit smaller than the production batteries.
There's a simple board that spans the top of the batteries and a threaded rod between the two. A wing-nut holds it all tightly together so there's no wiggle room when it's all clamped down.
The batteries are now mounted, but I'll probably have to haul them out one last time when I put the motors and main wheels back in.
I also have the circuit breaker and auto headlight
"kill switch" mounted right below the charger power connection, so the basic circuit is coming together.
I'm working on the motor rotary encoders next.