After a mock up with cardboard, I fabricated the aluminium battery box base and screw it to the body.
I do enjoy working with sheet metal and its workspace in FreeCAD. I also invested a bit in the Technical Drawing workspace to send nice instructions to the laser cut and bending guys:
Previously to installing this box, I test fitted the battery pack alone by hoisting it into the boot. You can also see the HV cables are cut to length and have a big fat fuse.