The bathroom door tends to move in a half closed position. As such, the vacuum robot will be able to enter the room by gently pushing the door to the side, but will be unable to leave the room again. I used a door stopper for a while, but got annoyed by constantly wedging it in and out under the door.

So I came up with this design. It is specifically dialed in to the distance between the bathroom door and the storage unit. The hook is held in place with a M6 carriage bolt. Hence the square hole on one side of the holder. The clamp on the other end fits the diameter of the door handle, which is 20mm. 

I include the Fusion 360 file, so you can remix and adapt the design as you please. I'm by no means a professional, so feel free to improve on the design. :)

Happy Holding!