Suzie, Sorry, I didn't see your posting here earlier. How easily they fit, and whether they stay in depends on the internal diameter of your urethral tube. As you sound more and more, the spongy tissue gradually gets more compressed and does not go back to previous normal. This happens even if you're not trying to stretch it. Also, it depends on the rhythmic routine used, and how much power is applied. Some of the ET-312 routines tend to cause rippling contractions that tend to push the trode out when used on higher amounts of power. In that case, you would have to hold it by the thick part of the wire so it doesn't pop out (which results in a sharp shock). The shaped Cockapedo tends to stay in easier, since the small sphincter in the urethra near the head of a man's penis fits in the narrow part of it, and tends to hold it in. If I feel it coming out, I usually just hold it in by the wire with two fingers. I do the same when using a urethral trode on a woman.
Yes, I have used both of these. When I started trying these, I couldn't get the Cockapedo in -- only the smaller one, and it easily stayed in. After a while, it tended to easily fall out (it's fairly short). and I found the Cockapedo could then be inserted, but with some difficulty. Now, it goes in easily, and can stay in if the power is low, or the rhythm is more of a vibratory one rather than strong pulsing. It can also help to put a narrow diameter wide rubber band around the head of the cock just below the glans.
It's an incredibly fantastic feeling for a man. I've also done this with two women who previously sounded themselves regularly, and wanted to try it with electro running. One said it felt like a continuous orgasm for the two hours; the other also had several orgasms and said she loved the feeling.
Four years ago, when I was just starting to learn about estim, one of the first things I read was a blog written by a woman who had tried estim with a guy for her first time, who put an electrode in her urethra, among trying many other things too. At the time, I didn't understand how unusual this is, and that almost NO women in the US have ever tried this. I've tried to find that blog again, to reread it with more knowledge this time, but I've never been able to find it again.