The E-ring has a screw attached to it so that it turns the meter prong as the aperture ring is turned. This screw is not visible with the lens mounted because it is hidden by the shroud of the ring, which also protects the meter prong. The meter will not operate until the E-ring has moved the meter prong about 2.4mm clockwise (looking at the lens mount). If the lens is mounted with its aperture ring in full open position, the E-ring moves the prong about 2.4mm. As the aperture ring is closed, the E-ring pushes the prong further. So all that is needed is something attached to the aperture ring that will push the prong. It must of course be in the right position.

In the case of the 50mm and 80mm lenses, the aperture ring is close enough to the body that a little block could be glued onto the part that is very close to the body, and hence be able to move the prong. I have attached a photo showing a body without lens and a label showing where the prong has to be moved to when a lens is mounted while set at full (widest) aperture. It moves the prong about 2.4mm, and then further at smaller apertures.

This method of avoiding purchasing an E-ring only works with the short lenses. The longer lenses have a non-rotating part near the body as their aperture rings are further from the body. Consequently their E-rings are deeper, culminating in the E-ring for the 250mm Sonnar that's about 45mm deep.

I hope this makes sense. I would use a little block of aluminium or something hard and easy to cut to size, and glue it with a tiny amount of araldite or E-6000. I was going to do that but I managed to get several E-rings. I already had a lens with an E-ring fitted, so that made it easier to fit them to the other lenses.

Please let me know if you'd like any further info.