Wondering about the design of the case.
You could actually design 2 different cases.
1 for day to day use. At work I only really need the 2
batteries, and probably 2 carts. (Made one from an old hard side eyeglass case)
The second one as a travel case.
If my wife, and I are going on Vac.
Man, we would need to carry all 4 batts, a charger, or 2, Maybe some tools for refilling, and of course some bottles of
juice.
Of course you want them to be fairly tough so you don't damage anything.
Maybe some loops of elastic, and fitted for batteries, and carts, so things don't just roll around all over the place.
Anyway just some thoughts on it.