I have made a mounting system for the 4 deep cycle batteries.I made a plywood base that is the same shape as the rug, so it fits snuggly on top of the electronics box.This assembly will be held down by nylon straps.
The plywood will distribute the weight on the electronics box, and the position near the midpoint of the car should distribute the weight more evenly over the front and rear wheels than placing them in the spare tire well(would not fit there anyways). The big surprise was that when looking out the rear view mirror, I can't even see the batteries, and have the full view out rear.
I mounted a 50A 600V connector in the lower passenger side of the electronics box, to allow easy connection to the car. As presently wired, the boost current will pass through the MPI current sensor as well as the battery SOC current sensor, and will only charge when the car is turned on. This is where I connected the other day for the first test run, so I will explore the cars reaction to boost charge via this path before moving the connection to the other side of the MPI current sensor.
Next will be to get the boost equipment mounted for more testing.
I was able to completely charge the 4 batteries with my PV panels during a single day of full sun.

Opps! The extra weight has dropped my rear suspension to within 1/2 inch of bottomed out, without me in the car. Afraid to drive it that way. Will need to look into beefing up the suspension.