Put the rubber on first, and then insert the string. I think it needs to be big (like clothes line) to avoid cutting the rubber. If you need to, tape the rubber in place in place with masking tape. The rope will cut thorough the tape. You can use linseed oil as a lubricant, when it dries it turns to glue.
Number one rule: don't get mad!!!!!
With gentle pressure from the outside, it should get about half way seated. Use a cotter pin removal tool or not sharp screw driver to help the rubber along from the inside. NEVER pry against the glass. If the inside lip of the rubber is over the metal all the way around, you can seat it by slapping it. Take off all rings, and do it in such a way that it stings your hand. If your hand doesn't hurt, you will break the glass. Remember rule number one!
I'm still in the business, working for a manufacturer (LOF), and happy to give free advice (worth every penny!)
Leif the Old Glass Guy 415 259 0354