I have used two inflatable things for stretching: a bulb syrings (ca. 3" diameter) and my own proprietary inflatable pump, normally used to injecting semi-solid enemas. Toget the bulb of the syringe in, I first relax my sphincter with two or three fingers lubricated with soap. Then I cover the bulb with soap, squeeze it flat, double it over and insert the whole of the rubber bit, leaving the vulcanite pipe outside me. I then 'suck up' my abdomen, lowering the pressure in my rectum so that the bulb pops out to its full extent.
Next, placing a finger over the hole in the pipe, I bear down slowly and in stages to expel the bulb fully inflated.
The pump is a piece of 2" diameter plastic waste pipe clad with a rubber sleeve made from a motorbike inner tube. I can insert this with the sleeve deflated and pump it up once it is inside me. This is very exciting! It can reach such a size that very great effort is required to expel it (harder than the 3" bulb!); but of course it IS rubber and does distort to allow it to come out without fail.
My greatest fun is to insert fully into my colon a butternut squash. I choose one about 3 1/4" around the big end, and if possible 8 or 9" long. After stretching as above, I lubricate with lots more soap inside and outside, and coat the squash as well. Then I rest the thin end of it on a closed toilet seat. Bearing down as hard as I can as, though to take a shit, I lower myself on to the squash and guide its great head into my anus. If I'm porperly stretched, it goes past the fleshy lip of the anus without bother, but then much more pressure is needed to get it through the muscular channel between my hips. Once past this, it is firmly embedded in me, and it is VERY difficult to get it out again.
But this is only part of the fun! I now make it function like a fist. I push it much further into me with an object like a long shampoo bottle. First it slides past my prostate, with delightful sensations and a gush of prostatic fluid from my penis. Then , with a bit more effort, it enters the descending colon. I can push a long one right up under my ribs where it makes a large protruberance of flesh! If I withdraw the 'pusher' the squash will stay there for days. It may want out, but without proper technique there is no way I can shit it; and absolutely NO chance of being caught 'needing to go.'
To get it out again, I press backwards on the bublous head of the squash through my abdomen. As it begins to move I can feel hte stirrings of a need to take a shit. Then I press downwards and the narrow end moves into the rectum again. With a bit more pressing, my own internal muscles take over until a good bit ot the narrow shaft of the squash is sticking out of my arse. But it will not come out no matter how much I bear down unless I also grasp it and pull it through the constricting muscular channel. It comes through with a rush, and at that stage I have to restrain it or it will split the skin of my anus. Finally, I can ease it over the tight rim and get it out.