In my picture above, the battery tray is mounted inside an angle iron frame which is bolted to the floor above using carriage bolts.
If you have room under your floor in a nice area below your current battery, and also have a body skirt, maybe you could make two vertical cuts and use a piano hinge as mine has, then come up with some kind if latching system. This would allow mounting a pair of batteries below floor level, maybe right where the cables are located.
Just throwing out ideas without knowing your particular situation.