I recall a car that did horns that way, i think it was a forbid or something. Not sure I'd want to blend the midbass with that... I played a lot with large diameter compression drivers but never found the right vehicle. I talked to someone with an old Studebaker pickup about going that route.
I've got farther to go to integrate with the car interior. Results are promising. The stage is good all around and imaging is solid and focused with the xover / phase adjustments I've done. Its good enough that I'm excited by the potential and how the plan is working out yet happy enough to be less motivated to finish.