Hold it closed as tightly as you can, then put a strip of tape across the bottom of it just above the window trim.
Then keep stacking layers of tape on the next until the tape is too thick to allow the window to slide down. I would recommend doing this on the inside of the car and not using the passenger side door if at all possible.
The closer you get the tape to the bottom of the window, the better the seal you should have... plus it should be easy to clean the glass to get the residue off. The only other way I think you could do it would be to just take the door panel off (gonna have to do it anyway) and then rig up something to prop the window open that is a more secure option...