The rubber eye cup is set into the socket in the clip. The clear diopter is only needed for its threads. It affixes the rubber cup to the clip. The clip with assembly is then pushed onto the camera viewfinder.
I threw a little dust on this so you can clearly see the glass diopter. It has a mechanical function only. Not for optics, so get a 0 version. Take note of the diopter threads.
This store in Hollywood sells the clip & eyecup. These guys are big guns with the movie industry. They have two magnifications in dioptors. But not the + 0. With the wrong dioptor, maybe the adjustable dioptor on the camera itself might compensate for it.