And what about the car behind him? He's now up on your tail, wondering wtf is wrong with you and why are you blocking a spot he wants... The only reliable way

And what about the car behind him? He's now up on your tail, wondering wtf is wrong with you and why are you blocking a spot he wants... The only reliable way to do this is to stop before you pass the spot, have your passenger get out, and then the passenger blocks the road and pretends to give you guidance on how to back in. That makes it obvious, and it also physically prevents them from stealing the spot unless they're comfortable with driving over a human.