[pb4ugo]

I've been playing with this idea for a couple years, haven't built anything yet, though. I think bottom access is best. The body would get in the way no matter how you slice it and dice it otherwise. That is a non-critical area of the chassis, and both the cups and the chassis braces would provide all the rigidity needed. If worse came to worse, you might have to run the alum braces instead of the plastic the team drivers run.

My idea was similar to yours. I was going to have a beveled lip on the end of the tray opposite the bullet connectors, and engage that lip, and push the front of the tray in place. I was going to have a post on the front of the tray that would engage with a mount in the cup and then lock it all together with a 6mm by 50mm ball release pin. Pit stops would be reaching under the body, pulling out one pin and dropping the battery tray, replacing with another battery tray with the fresh packs, replace the pin, and go.