I've seen batteries mounted to the back of a mesh seat, which would seem eminently fabricobble-able, and decent enough weight-distribution wise. P-clips and a bit of sheet metal, with appropriate drillings for the battery bracket, or something.
Natalya's ~12.5kg battery[1] on the rear rack (I molished it into an off-the-shelf quick-release rigid rack box from Rixen & Kaul or similar) has a significant adverse effect on handling, but it's acceptable as she rarely rides faster than about 20mph. Better rear-wheel traction is a useful side-effect. More significantly, it means you can't use the rack for other luggage.
[1] 820Wh of LiFePO4, which made sense at the time.