I had learned a lot on long thread here: The battery builders club - How-Tos & Esk8 mechanics - esk8.news: DIY Electric Skateboard Forums Most of those guys like to build really clean batteries.
I haven’t seen a written up guide or anything, but you can read through there and even post your battery build.
So not to fill up nice2cu’s thread. As far as efoil packs, I’ll make a new topic here, Efoil Battery Builders Club