ah, ok. Thank you for the clarification.
It never occurred to me to 'reuse' the M5Core2 base plate to complete the stack. It probably won't fit under any of the modules as they are all quite different regarding where components have been placed. Plus most (all?) modules are lacking a connector for the battery anyways. So yes, the question remains as to why it has been designed like that. I guess only M5Stack engineers will now.
Yes, the M5Go Battery Bottom 2 could be a solution. Please be aware though that since it contains additional hardware, it consumes additional energy. (The 10 LEDs alone eat about 1 mA each, even when they are off.)
Maybe it's time to start the 3D printer and design you own?