my diy pack is actually a toolpack battery from our garden centre in the UK. a Macalister pack. They are 36v pack, made of 20 18650 cells. Rated at 2.6ah I used 4 of them on Saturday to complete the "diggle jiggle" with my Husky and a couple of friends. Next time I have my hernia fixed and will do it on a non-electric bike as my friends were giving me a lot of stick for the motor and the dog.
Anyway I have a plastic printed holder which is underslung on the downtube and allows me to hot swap the packs. I am finding this pretty convenient. I like the form factor of it, There isn't much wasted space or extra weight since they are plastic shells. Plus now I have the design down for the battery I can print more to allow all my bikes to use the same batteries.
In the future I may re-cell them with higher capacity as I wont need to buy new chargers either. I've a couple of dead packs that I bought that way for experimentation.
I really like your NESE idea, I hope it takes off. I may use the old but not dead cells in your packs in a second life scenario not on the bike.
Theses packs also come in a 4ah variety which I believe is just different cells inside so I am guessing its a safe bet to just replace the old cells with better ones in the future.
Sorry for the long post, hope some of that was helpful.