The diaphragm isn't a seal. It's just an oil barrier. The rubber pad is about 5mm in diameter though I cut the replacement more like an octagon. The shape isn't really critical since it's function is more of a snubber for a small tube at the end of the vacuum supply line.

Here's the VCV off the IP. I gently drilled down the rivet flares to remove the plastic case from the metal cover. This isn't absolutely necessary if you can remove the orange cover. I couldn't remove the orange cover (mine's black) with the metal cover in place -



Here is is the metal cover off the plastic case. The diaphragm doesn't seal anything. Not with that big hole for the vent fitting anyway -



This is the plastic case with the valve mechanism removed. Nothing to do with this other than clean it -



This is the valvetrain. I think things are in the right order The metal tube at the center of the orange/black cover presses against the rubber pad a couple of pieces down to hold vacuum. As you apply throttle, the pad drops from the tube to allow a leak. In this picture the rubber pad is worn down to an o-ring. It should be a solid disk -



Here is a picture of calipers showing the diameter of the rubber pad. As I said I cut a piece of rubber closer to an octagon and it works -



Here's a picture of calipers showing the thickness of the rubber pad. The bicycle tire inner tube I used for the replacement pad was close to twice as thick as the original pad. Doesn't seem to be a problem -



I found that common household wallplate screws have the right thread to self tap into the remaining rivet sleeves. It looks ghetto but you ghett what you pay for.

Sixto
87 300D