Just an opinion please understand but personally I'd not trust your eye screws and bungee cords to securely hold the batteries in place. The eye screws could easily be pulled loose from the plywood, bungee cords can fail without warning if not carefully and frequently checked.
Your arrangement will work for the short term but I'd re-think how you're going about this. Otherwise good work and I'm sure your van is a lot of fun AND useful on trips!