Although I am 75 miles from the Jeep, I have my computer and Sketchup with me. So, here is a model of the under-seat box and rack. I'm sure of the box dimensions (the battery is the same as all the other batteries...) but I am not sure of the rack dimensions - I will need to measure very carefully.
So, here is a rendering of the box in the rack:
This is just the rack - conceptual at this point:
And this is the sheet metal pattern for the box: