I've been trying to figure out how to describe my idea to you...
If you can imagine something about the thickness of the prop rod under the hood of a car: Now if that rod had a ninety degree bend at one end (with about a half-inch or so stub, and a hole for a cotter pin...
drill the appropriate sized hole for that stub to fit through in the bottom edge of the door (
) place a washer down over the stub, put it through the hole, and then add a washer to the outside part of the stub, and the cotter pin...
The rod should easily lay up on the inside of the bottom edge of the door... you just need to figure out how long to cut it, so that you can prop it against something solid. holding it up on the door edge should be an easy thing to figure out.
Oh! One more thing... have it positioned so that the prop rod lays with it's sutb end at the front end of the door... if it were to drop loose while moving; it'd probably do a lot less damage when set up like this.
That's my half-baked idea... I hope that you can make something useful out of it.