As the body does not have to be road-going, you might find that weight and structural integrity are not as important as they are for a car. Even a bubble car shell has to be strong enough in the right places to take the many forces imparted by axles, wheels, steering, braking, and engine loadings. If your body is just fixed around its base to the boat, it should then be rigid and strong even if it is only an ultra-light-weight structure. Therefore it should be possible just to get a thin fiberglass shell, of just the exterior shape, without engine bay, wheel-arches etc. Thus if you find someone who is restoring a bubblecar/micro-car a mould could be taken of its stripped body, then a thin (1 or 2 layer) shell made from the mould. Once you decide what make of car you want to base your shell on, you could contact clubs and even Alan Hitchcock (alan's unusual autos) to see if anyone has a likely car in nice shape but not yet painted or assembled.