On my NPS I built a steel frame (1" rhs) that goes around the existing start batteries and out flush to the edge of my subframe. I clad it with 2mm Al checkerplate. It has room for other messy stuff as well. In my efforts to keep things symmetric the house batteries are in the equivalent on the drivers side (RHS). They are 320AH@12v LiFePO4 but about the same weight as the starters.
The build has progressed a bit since the pics but life keeps getting in the way.