I just bought an Optima auxiliary battery D34/78 Yellow TopT  and installed
it under my sink. Photo attached. (Can you attach photos on this list?)

There is no way anyone is going to install a battery behind the driver's
seat because of the steering wheel in the way of turning around the seat to
properly expose the battery compartment. Unless you radically mutilate your
vanagon.

I made a platform out of two 3/8 inch pieces of plywood 9 inches wide. The
top one has footprints cut out for one of those black plastic battery boxes
you get at RV dealers and I screwed it to the bottom piece which is custom
fit to sit on the shelf. The cut out template keeps the battery from
shifting but I also made a strap to tie it to the wall behind the driver's
seat. This makes the battery easy to access and service.

If anyone is seriously interested I will send photos of the installation as
soon as I get my digital camera back from Polaroid repair.

I am loving cherrying out this new purchase.

William