NOTE: If you are not fecal incontinent, the following is TMI.
The anatomically correct enema nozzle I use is a retention nozzle from siliconenozzles.com. Be advised that we will not find these items in medical supply shops. I have tried that and failed. It took me about two years of looking to find this one. The source sells a staggering variety of enema nozzles intended for the fetish market. However, they offer several "retention" nozzles, of which this one is the best (at least for me).

Just ignore the fetish items and look for what will work.
The benefits of a retention nozzle are that it reduces or eliminates leaks and it helps one retain (hold) an enema solution. For those who do not know, holding the solution for a period of minutes increases the effectiveness of the enema. I also use mild Castile soap (Dr. Bronners's) in my saline enema solution. That causes peristaltic cramps that cause the procedure to be much more effective (you do not have to do this), but more difficult to retain for the recommended 10 to 15 minutes. Do note that as I am taking the enema to prevent fecal accidents, I need to clear the colon much higher than normally is the case for ordinary constipation. That is why using a retention nozzle is so helpful to me. Also, as I have some nerve damage to my sphincter muscles, I need the plugging effect of a retention nozzle in order not to leak excessively (or worse).
My nozzle is listed as the "4.75” Oval Flange-Style 1.6” (41mm) Small Diameter Enema Nozzle." There are others in other sizes.
Note that the base of this nozzle is a T-bar shape so that it fits between your butt cheeks without poking into the sides. This means that you can use the pressure of your cheeks to help keep the nozzle in place during cramping instead of placing that strain completely on your sphincter muscles. The nozzle looks large, and it is, but once inserted the shaft is about the right size for the sphincter to contract around. I insert the nozzle slowly and with a generous amount of lube. Once inserted, I find the nozzle to be neither comfortable nor uncomfortable as, once the anus fits into the valley of the shaft, the slopes on either side cause a snug fit, as it must in order to be effective.
Also, these silicone nozzles are available in a variety of "softness." A semi-soft silicone formulation should be more comfortable that a completely rigid nozzle.
Other similar nozzles from other suppliers also should help, but getting the T-bar base is highly recommended.
The URL is:
http://siliconenozzles.com/475-Oval-Fla ... e-OF-1.htmOf course, as we all are so different and our medical situations are different, I cannot guarantee that this nozzle will work for you. However, IMHO, it is worth a trial. Good luck to all. If you obtain a retention nozzle, please be brave enough to give us your feedback, either positive or negative so that we all will learn.
--John