You cant' really fit a horn into a trunk, or even a car for that matter. All you could build would be a quarter wave resonator with a tapered mouth, why car audio guys call that a "horn" idk. Anyway I'd just do a ported 15 facing the trunk lid. KISS (keep it simple stupid) Unless your box builder knows how to use akabac or hornresp, he has NO idea how a enclosure with changing cross sectional area will model. Even if he does, modeling it in a car is tricker still.