Tonight I also played with Sketchup and created a template for the sheet metal enclosure that will surround the lower rear rack. This will keep water and road debris out of that battery pack. Here is what it will look like installed:
And here is a template that I can replicate on two 24"x48" sheets of 20ga steel. Just need to cut, bend, and weld:
In addition to waterproofing, this should be a good additional support against fatigue cracks in the rack.