Just use a flat plate on the back of the lens. A small rectangle or square with a screw through it.
double purpose it one side for f8 the other for f16 or whatever. It would just be different lengths on each side.
There will be plenty of clearance using a flat head, countersunk screw. Hey, if you have a scrap lens make the stop from an old stop down lever.
John, you cannot use the aperture placement as you suggested because it is not INSIDE the lens. I was under the impression that the planar location of the aperture is VERY important. - David Lyga