I have one that I put a Clark on, but it didn't sound like yours. The crystal that came on it fit tightly over the crystal seat and then the retainer pressed on just like gen. When I put the Clark on it it was just the same. There was no extra piece of plastic and it needed no custom ring.
If the case is the same as it was eight months ago it sounds like you do not have the proper crystal.
Also, in the rep world, you cannot usually mix and match bezel rings with retaining rings. It usually has to be a set, unless it is a true Rolex replacement case.