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.

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

Road traffic laws are absolutely not harmonized across the EU.

French, Portuguese and German rules of the road for example are all different in statutory strictness and enforcement strictness.

I’d guess that’s true in the US too, if you modify the engine or transmission or exhaust system, etc. But adding something to your car without modifying the existing systems has no regulation and no certification process, which he sorta talks about in the article re: police inability to enforce. As long as there aren’t other laws you violate, hanging a big fan out the window might not count as modifying the car.