If it were me, I would throw the metal part in the freezer and heat some water (microwave or stove). once the water is boiling, pour it into a bowl and soak the rubber to make it softer. get everything ready and setup to put together without delays.
pull the part out to the freezer and CAREFULLY seat the diaphragm onto it. use something small and non-sharp to roll over the thick area and push it in place. Don't try to yank it on by stretching the thin fragile part.
That said, I've never done this exact job for this specific part.
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