If you have seen my other post, you can see that I mounted my Lastolit Ezybox on the door. Yes. I mounted it on my door, not a lightstand. I did it with a Manfrotto Super Clamp. This is a "massive" clamp. It is much bigger and heavier than I expect. I used it to clamp onto the door, then I attached a light stand umbrella adapter to it and mounted the Ezybox and the flash on it. The super clamp is strong enough to hold everything tight!
Here is a picture by itself.
Here is a picture when everything was disassemble. You will need two studs. One stud to connect the super clamp to the umbrella adapter and another stud to connect the umbrella adapter to the Ezybox. Those are just standard studs.
By the way, it's easier to mount the super clamp on a flat surface, i.e. a door, when you have the little wedge, shown in this picture. When you buy the super clamp, it doesn't come with the wedge. Some doesn't even come with the stud. You can get the wedges from here. The part number is 035WDG. It comes in a set of 4.