It sounds like the panel[s] you choose would be a good fit within the roof rack frame. If you put quarter-round strips down the side, a bull-nose strip across the front and a partial boat-tail across the back you'd have the aerodynamic blister you want.
18650 is a cell form factor. They are used in laptop batteries a half-dozen at a time and in the Tesla using thousands in a pack that takes a forklift to move. Tesla cells have an extra thermal vent or something.