Since it's just for range extension, you could maybe stuff the engine in the rear, so you don't have to pipe exhaust under the whole car. Although then you probably want to pipe coolant to a radiator in the front and have to protect that piping... but at least it's only coolant hot and not exhaust hot. The engine noise would be away from the driver at least.
You said you didn't want the heat of a gas car, but you might choose to run it a little bit more in the cold to help warm the interior on cold days (especially at startup).
Potentially, but not really likely, you could try to make it modular-ish... attach it when you need it, leave it in your garage otherwise. But I think it's going to be too heavy ... I have a 7200 W portable generator which is roughly half of what an EV uses at cruising speeds, and it takes two people to lift it. Automotive engineers do amazing things, but I don't think they're getting twice the power and less weight and easier to move...