A little sidenote:  I had been considering developing a similar robust box underside Vanagon, between the frame rails, on Passenger Side, just forward of motor.  The idea was that it would be used for "rarely used" but "wonderful to have" emergency items.  I used Solidworks CAD to design it; I thought it was swell (like any inventor)!!!  But I didn't have the gumption to actually turn it into a product and try to sell it.  Instead, I made a hefty plywood box to achieve the basic same purpose; even with a locking key latch.  Works quite well: but the only thing I currently have in there is an empty 1 gallon gas can, a woven nylon tow strap, and I think some 3/8" siphon hose should the need arise.  But Dennis, not only can your design be used for a battery, but for someone else that utilizes House Battery under Driver Seat, a great storage compartment for Emergency Use items.  I love your hinged access design.

    On Wednesday, June 9, 2021, 4:03:27 PM PDT, Karl <tdiguru@westyventures.com> wrote:  
 
 Lifeline told me it wouldn't be a problem using them as starting
batteries, so they were combined as one, with no starting battery. It
was a TDI and started easily. They were in use for 8 years in my
ownership and two successive owners of the van, one of which toured
Latin America for 21 months.

Custom Li batteries are available, a friend in the solar / battery
systems industry ordered and installed a couple in several Vanagons in
the stock box under the driver's seat. 130 Ah.

On 6/9/2021 1:51 PM, Dennis Haynes wrote:
> That is also a great option. To keep the 2 100A/H batteries isolated to be used as dedicated house batteries a standard battery with vent tube can be placed right side under rear bench and connected to starter.
>
> It would have been nice if the battery box was just 3/4" taller so that Group 48 batteries would fit as that case is available in AGM and LifPo
>
> Dennis