Not a butt plug, but a vegetable - a butternut squash. These have a bulbous end, but where the stem was attached the fruit narrows considerably. I can stretch my anus to admit something 10.25" in circumference (260 mm) which works out to an equivalent diameter of 3.25". This is really hard to get in, and I must spend quite a lot of effort relaxing my sphincters (outer and even more importantly, inner) to get it in. A good butternut for the job will be between 5 and 9 inches long, and I put it in wide end first. It must go in on top of a really BIG enema, because the water must force it against the anus from the inside, or else it leaks.
It makes a superb plug, because if I'm standing, I can't get it out at all, no matter how hard I bear down! The narrow end hangs out of my anus, of course, as a kind of opposite to the flared base of a conventional plug. But where the ordinary one wants out, this one stays firmly put until I squat and bear down! I have to do even that quite carefully, because it is painful coming out, and I really want to give the sphincters plenty of time to relax and let it come.
For a variation I can sit on the protruding end of the butternut (e.g. in the shower) to force it further into me and the water will force itself out round the narrow end. Nice prostate massage, too!
If you aren't that stretched, don't stress - find a smaller veggie and have fun with that!
Johnny