Your requirement of having the boat tail stowed in the car was a challenge. We had earlier concepts of rigid or semi-rigid boat tail appendages mainly supported by extended hitch connections. So a folding inflatable could be attached ala bike rack straps & a rigid spar to hold the rear lights/license assemblies. The inflatable form could theoretically have the optimum airflow shape. Inflatable aerospace tech has been updated from earlier Goodyear airplane studies. The exciting 'Mars airplane' studies depend heavily on reliable inflatable airfoil engineering solutions. The tech has already been used in the Pathfinder projects.
Goodyear have taken early tests & concepts of inflatable airplane platforms. A compact rescue package designed for downed pilots/operators that could be air-dropped. They could then be deployed & flown to their freedom and back to friendly lines. However watch out for catastrophic deformation at extreme speeds.