The mounting is clear to me, but what is the diameter of these ear cups?

The product photo on the right shows the ear cup on the mounting side. The thin plastic ring has to be slipped over a ring on the headphones, which requires quite a bit of dexterity and is fiddly. I cannot imagine that the ear cup would fit on a third-party product that does not have exactly the same dimensions. On the Sennheiser headphones, the replacement ear cups fit like a glove.