Well not sure where you are, but where in EU I am, you can modify the insides of your car (people attach entire apartments to the insides of their cars...), and there's nothing wrong about stuff sticking outside - you just need to attach a red flag if it's over 1.5m out of the car (maybe red flame would be enough?).

The modified rear door might be a problem, but where I am you could simply keep the original open, or detach it.