I remember doing those in the first STS. The tubing Cathe used to sell came with a door anchor. It was a flap of thick canvas with something like a marble inside at one end and a loop at the other. You’d hook your tube in the loop and put the canvas strip on top of your door, shut the door and the marble would hold it in place. I knew it would stay put because I’d even put my feet up against the door for more bracing.