Okay, I’ll be the voice of caution here.
I once was mentoring a team that elected to put their battery mount upside down as you’re proposing, and we had an inspector who would not let it fly. I was involved in the decision for how to mount it, and I know it was rock solid. The whole depth of the battery was supported—basically five full faces of the prism surrounding the battery—it did not hang below the frame, and the bottom side was supported with two redundant straps. But nevertheless, our inspector didn’t like it and wanted it changed.
I’m not saying I think it’s a bad idea, but be prepared that some inspectors may not see it the same way.