Break in the flying season. 
Pulled out all my velcro battery straps and started working on a new idea. I want room for eight. I want to keep all the wires short. I think they’ll fit if they’re stacked four across and two deep. Made a shelf.
I’m using the 15/16" holes just below centre to take the weight. They’re spaced really nice. These plastic tubes are for proof of concept. I’ll use aluminum of maybe CF.
They support the shelf which will be an aluminum or CF channel. Not sure if I want to tape the batteries together into a single brick. It’s a lot of weight to handle.
Batteries clear the motor arms and props nicely. I have a piece of white foam in the gap to show it.
The motor arm swing bolts are in perfect position to anchor straps holding the “brick” against the frame. Just velcro for checking the concept, will switch to some hold down straps.
It seems to be pretty solid. I’ve put it on. The higher cg is ok. With four more batteries on a bottom shelf it’ll be lower again.