youdontknowme I am sure Mr. Tollyboy has some interesting stories about overconfident clients who believe that problem to be solved.
Yes, there are many who think this is not a problem. Take a two-inch medium wall thickness tube cut a thread on the inside to screw in a bung weld it to the back of the belt and call it good. Apart from the problems of how would you insert it with no tapered guide on the top to ease it into place, how are you going to deal with a permanently overstretched anal sphincter, why put threads on the inside of it where they will be harder to clean. etc. Anyone wanting to go this route needs to consider the possibility of permanent anal incontinence so even if you get over this fetish you might well be wearing adult nappies for the rest of your life or may need surgery to insert some form of an artificial sphincter. I have had lots who want this with no consideration of the consequences of it.
The only permanent anal plug that I have so far ever built and that I know works is made of half-inch bar and is turned much like a dumbbell. There is a half-inch ball on the end that is inserted the stem is small so causes effectively no stretching about 4mm as I remember and there is a ball on the other end that is drilled through and soldered to the rear cable of an FGA/200 belt. Round end to avoid cutting or tearing anything, very small shank to avoid stretching anything, not removable but not I imagine the cleanest thing to use. Would I think almost certainly need a bidet for washing?
The purpose is to be just big enough to serve as a reminder it is there and to prevent any attempt at moving the cable sidewards to perhaps allow some anal stimulation. I have no idea if it actually works but then I rarely get feedback.
