Moliere---in answer to your questions: I use the Siliconnozzells large diameter (I think it is a CT122) all of the time. It is my favorite. It will not stay in until you get it past one or both of the big bumps, and to be certain it will stay, I took some off of t he diameter of the valley of the last one so that it is perhaps 1" + or - and it will not come out on its own, I have to push it to come out, which is nice for the feeling of security it gives as well as a better seal. I would tell you that my experience is that getting it in past the two last, large bumps requires a good lubrication of the nozzle or the anus or both, which can be difficult if you are working by yourself.
When I take the last bit of the tip out, I do not just take it out or I will leak. I take it out slowly for the last inch and hold it against the anus when it is all of the way out until I feel that the sphincter is completely closed. Then I do not leak.
The CT122 is expensive, but I am glad I got it because it works so much better than the usual 36fr colon tube and it is much more exciting and satisfying than the usual one also.
I also have a 22" nozzle not from Siliconenozzle, but I do not remember who sells it. The large end is perhaps 2" and the small end perhaps 1" in diameter. It has veins over its length, which is stimulating. Both ends are circumcised glans. If I put the small end in, it will not stay in, but if I put the large end in, it will stay wherever I put it, which is nice. I generally put it most of the way in, leaving perhaps 4" out and then can fuck myself with that length, which is an enjoyable thing. Again, when I take it out, I hold the last little git against my anus until the sphincter is closed or I will leak.
I have several other nozzles and tubes, but these two are my favorites and if I am doing two or more enemas in a session, I alternate them.
hankb