Does a polished stone ball count as a buttplug?
Obviously there's no 'neck', nor a base that remains protruding.
I've got one. It's 61mm diameter, and weighing nearly half a kilogram, and I reckon it must have spent something like three years inside me since I first found it in the window of a charity shop fourteen years ago. I use it whenever it might be inconvenient or embarrassing to have the base of a real buttplug showing at my back door. I love the weight of it, plus the fact that it's so easy to insert and hide. With lube of course.
So with this one, I hardly ever feel vulnerable!