This came to me as I was doing some of my best work on the john.

one th sliding end of the stay, whiz wheel a square notch, about .125x.125x.125" just outside the sliding hole. Using a piece of 1/8" stock, cut a piece about 1"x.25", and drill a hole about 1/8" off the end. Put a self drilling screw just smaller than the hole into the pice of stock, and position it where you can swing it into the slot in the stat, then drill the self driller into the side of sliding hole in that position. when you want to prop the door open, swing the tab into the slot.