I used the POLOLU-3752 driver board with the Pi 3, and modified the code found here -
turning it into a ROS node so it could communicate with ROS nodes on the Xavier and connected the Pi to the Xavier via Ethernet. The driver board is powered with a 7.4V LiPo battery and the board supplies the power to the Pi. The Xavier has a completely separate power system using an external laptop battery. I did design and 3D print a small enclosure that the Pi and driver board sit in one of the lower open compartments and used the other compartment for the 7.4V LiPo. The external laptop battery sits on a mezzanine platform between the top plate of the WildThumper and the chassis consisting of an aluminum plate connected to the lower chassis with standoffs and with a rubber surround on the side of the aluminum plate to avoid damaging the tires if they bump into it. I have designed and 3D printed a number of different mounts to mount cameras to the upper plate of the WildThumper which is also where the Xavier is mounted. If you want to do something along the same lines I would recommend you have access to a 3D printer and learn how to do some CAD so you can mount things nicely, I used OpenSCAD as it’s just a different type of code to learn, though you could do the same thing with wood and some hand tools. Currently the system is used indoors, if you want to use the system outside you’ll need to think about shielding everything from water and dirt and also dealing with cooling. I hope this helps somewhat, good luck with your robot.