On my Canonet QL17 GIII I have fashioned a small vertical rectangle of black plastic cut from a Dwayne's Photo slide box. At one end a circular hole allows me to mount a standard Nikon diopter eyepiece. The plastic rectangle is then firmly (but not permanently) attached to the back of the camera door using small 3M double-sided adhesive squares. It's positioned so that the diopter lens is centered directly over the viewer eyepiece.
It's surprisingly robust and unobtrusive. It's never popped off in use. But it can be cleanly peeled away whenever it's time to change to a more powerful correction eyepiece. And because it's only attached to the door it does not prevent me from opening the back to change film.
It's worked very well for me.