I guess what I'm asking has anyone found an alternative to the standard boxes
If your standard Grp 24 box will accommodate the additional height of a 6V, the cheapest upgrade path is another single box of the same size. The next cheapest path is two single 6 V boxes. The most elegant is a box that will house both batteries and these polyethylene
boxes are the most durable.
Two 6V batteries from Sam's Club or Costco will be about the same price or lower than two 12 V marine/hybrid batteries from Walmart. At least that is the pricing in my area.
I actually built a dual 6V battery box and, IMO, your time will be better spent elsewhere.