Eric
Let me suggest a couple of things: I know of no clamp that can assist you. Don't have both hands lubed up, that will help.
First, the tube bore won't be any smaller than 7/32" but let's assume that it is a 54Fr tube. And at the smaller tube sizes, the softer tubes can be a bit floppy.
Without practice, there is such a thing as too much lubrication (and too much water). Granted, it helps a lot to be reasonably well lubricated and have some amount of water /solution within. But it can also be counter productive. I have larger tubes (75Fr and 122Fr) that will slide through, but I can also thread my 72-inch 36Fr tube through most times if I just want deep solution and not a lot of tube resistance. They all slide rather nicely with about 1-liter held internally.
Let me suggest the following:
A weak lube/water mix (I use j-lube, only 1/2 tsp/per quart/liter, but you can mix any any dilute lube combination to accomplish this). It only takes about a quart and no more than two. Do a quick cleanse enema with about a quart of lube solution. Anything that is in your sigmoid and descending colon will probably come out pretty quickly (I think it feels pretty wicked when the clumps are lubricated like that and speed through the lower colon and out). What you don't want to happen is have your lower colon so "stimulated" that the muscles are too closed down. And you want the residual lubricant to be there to help the tube slide through.
You say you can only get about 10 inches in before it wants to slide/push out? It is possible that even with that amount of solution within you and with lubrication that the sphincter at the top of rectum and the first part of the sigmoid is "closed." And that is really the first turn/restriction you've got to navigate and the tube will try to fold at this point. I know that at about 7-8 inches, I begin to navigate that "turn".
For me, it sometimes helps to "aim" the tube toward the middle of the back to start before trying to push it straight through my central core. If I feel resistance, I back off the pressure. Once the tip of the tube finds the proper location it will usually begin to slide through and open that set of muscles and it will slide in very easily. This is where having too much water is a hinderance because at that volume, there is likely about 15-20 inches of water pressure being applied by your colon muscles against the water and holding it while allowing the muscles to relax is not an easy thing to do. I know there is this much pressure when I'm full because there is a column of water about 15-20 inches above the highest point of water inside of me when the bag empties.
So let's say you've done the 1-quart purge that I've recommended. There may be a little liquid left and that's okay. You just want to have a relatively clear path into your colon. Lube the end of the tube and you anus. Don't lube the whole tube until you really get the hang of this.
Slip the tube in, and insert until you get to that first resistance point. It should allow the tip to slip through with some initial resistance. Don't force it if it refuses to go through. Back off the pressure slightly and maybe give the tube a slight twist and a very small amount of water. This is usually where the tube will slide in to the next point. From here on, it is going to be a learning experience if you aren't accustomed to sliding a tube deep inside you.
I have a "long" sigmoid colon. I read it described as being one of two types; long or short. Mine is long and when the tube passes into the sigmoid it goes a long ways up my abdomen before turning back down and toward my left side. If you were watching my abdomen while I was laying on my back you could see the tip passing under my skin. it turns back down for a distance before entering the descending colon. Usually this turn in the sigmoid doesn't present a great challenge for me but there can be resistance. If the muscles are trying to push the tube out, again back off the pressure slightly so that the wavelike motion can pass by the end of the tube. I find that the muscles will relax and the resistance to the tube passing immediately drops for the next stage of insertion.
I can feel the tube turn into the descending colon and if I prod my left side I can feel the tube passing up the descending colon. From here it will continue upward. The sensation is one of sliding through the sigmoid, less of a sensation in the descending colon until it reaches the next sharp turn; the splenic flexure. For me, at approximately 28 inches in, the tip of the tube is entering the region of the splenic flexure. The tube has to make a greater than 90 degree turn. Again, if you get resistance, don't force it. The further the tube gets in, the less the body's capacity to expel its total length quickly. The muscles have to relax to allow the tube to pass. If you get the tube to pass the splenic flexure, the tube should progress across the abdomen in the transverse colon. Again, a little water addition at these turns can help. If the muscles don't want to allow it to pass, don't force it because you've already got the tube pretty far in, For me, that path is back "down" towards my navel and towards the "top" of my sigmoid colon before bending back upward towards my right side and disappearing under my liver. It is a very weird feeling to having tubing passing adjacently and going in different directions. But for me, that occurs only with tubes longer than 40 inches.
All my tubes have retention balls except the 36Fr, so if I can get the ball into my rectum, there is less need to hold the tube in place. But there are times when I can only get any tube up to splenic flexure and no further so I have to hold the tube while filling. With a deep fill, I will turn onto my right side and feel the solution flow across my transverse colon into the ascending colon. With my longer tubes, I can try to thread the tube across my abdomen in the transverse colon and try to make the turn past the hepatic flexure to fill from the ascending colon back towards the rectum.
Let me also say that sometimes it is hard to get the tube in after the first maximum filling and subsequent fillings and it usually that point at about 8-10 inches that is tough to get through. A little liquid will generally allow the tube to pass, but not always.