it seems i have been preaching to the converted one
i too will look for a suitable replacement for the 'misplaced' o ring.
i think the easiest solution is of course a fibre washer it may mean having to buy a sheet of the suitable fibre and then finding the right size punches to make my own. not sure what the shape of the seat is in my isonic but if it is flat it should seal properly and as you know if/when the fibre comes in contact with water it expands thus creating an even better seal.
as for a cheap vacumn for green room, an old refrigerator compressor is ideal for pulling a vacumn, i use a glass jar as a water trap and this set up works well. i have to add i do use refrigeration gauges to adjust the rate of suction but a small sacrificial hole in the nose or tail should do as well.