The tailcone of this airplane has the optimal proportions for a boattail and is what I used as a model when designing mine. The Questair Venture is a 350mph kitplane nicknamed "The Flying Egg" for its short fuselage. The angle on the bottom side of this boattail is a bit sharper than optimum for a ground vehicle, but I didn't change it on mine since I needed the ground clearance for entering my steep driveway. Since your Volvo has a lot of overhang behind the rear wheel, it'll be more likely to scrub the pavement with the underside of your Kammback if the bottom angle isn't sharp enough. As far as the rear bumper problem, remove your stock rear bumper and design a new rear bumper into the back of your Kammback. Its not like they require 5mph bumpers any more, a bumper is just a flat peice of metal, foam, or plastic on the rear of the vehicle.
