Here's the start of my new battery box. I've finally bought an Optima battery which is around d the physical size of a commer battery. The one I've been using is a truck battery and very heavy.
The plan is to mount it as low as possible in the van, baring in mind that 90% of the space between the front and rear wheels is taken up with engine, gearbox, exhausts and roll cage feet there's only one place it can go... between the fuel tank and rear number plate. I've measured up and it will fit with only minimal modifications to the number plate panel.
I decided to make it big enough to store my tyre inflator and puncture repair gun as well as the front towing eyes. It'll also put more weight over the back wheels for traction.
Ironically I'm using old British Gas boiler covers to make a battery box for an old British Gas van.