Hi Kezo – I would have thought weight in a roof box maybe an issue too – ev’s by their nature are heavier cars – tho I think the Kona has done a good job of keeping the battery weight low slung under the car.
Might be worth posing the question on this and other EV forums to see if it’s been done and how it effects the handling – I have a cigarette lighter style twin usb for the front power out – but I cannot see why you couldn’t do similar for the rear power point for back seat passengers.