Long time lurker of the site, and I've learned a lot here so I thought it's time I give something back
I've measured up and found that 6s lipo packs fit very well in some rectangular PVC downpipe, so that's been the basis for my latest bike (1000w hub). PVC's cheap and really easy to work with and I think looks pretty good. This first pic is after I've cut the pipe to size with a cheap angle grinder, and attached an aluminium piece to close the top.
2 x 6s lipos fit snuggly in the pipe on top of each other, but still have a little room for expansion.
How the 12s2p batteries will fit in the case.
Velcro straps to hold the battery to the frame. It holds it really well without any movement while riding.
Added on/off switch and hole for the battery power cables into the other side of the controller, so power goes in one side and out the other.
Controller mounted behind the crank, trying to be stealthy
It hides ok from the crank side.
Completed bike, crank side
The next step now is to strip the whole bike again and paint it white, so it matches the battery pack.