Well it is probably maximum, but posted in other thread this: LCE930Will it fit to the frame?
Basically a tape wrapped battery without case, bms needs to go somewhere also. It needs to be a tight fit or have some kind of fastenings to stay in place. Your idea would probably be a more safe option and still lighter then oem battery.
I still don't undestand the can bus connection. If you make custom battery without that and it works, then why they use the can bus in the first place?