In my last build, I made the enclosure before the battery, and then predictably the battery didn’t fit without some workarounds. Was keen to avoid that this time around so had the battery made up before building the chassis which needs to contain the battery on a one wheel. Specs wise I wanted to go above what you’d find in a OneWheel and settled on a Molicel p42a 12s3p pack, of course then the GT was announced with a 18s2p pack. Same number of cells, different layout 
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Getting the pack built has been the easiest part of all this. Sent the following pic to @glyphiks .
And got this back just under a week later. Thanks again!
It then sat in a box for six months, but at least I knew how big it was and could start designing the rest of the board around it. Got lucky and found a FlexiBMS Lite in Australia of all places, and finished up the pack.
The pack was already built when I decided that I needed a wider wheel. This meant the board had to be wider and the battery pack width would have never been a problem. In fact, I think a 12s4p would probably fit, just not sure where the BMS would go. I’m really not sure a one wheel would ever need a 12s4p, but damn I’m curious now 