There is something about being completely immobilized, helpless, at the mercy of time and engineering. For me nothing heightens those feelings as much as being gagged and plugged, with much more emphasis on the plug. My favourite plugs so far have been the boring plain shaped ones. I have a set of them of different sizes, because the type of feeling I want from them changes as frequent as my mood or the weather.
One of my favourites is “The Barrel”, I had gotten it from extremerestraints and at its widest point, it is wider than a Coke can. It is so uncomfortable to walk with, and near impossible to sit with, so I generally only use it when finishing myself off, though even at its size if left in long enough it makes itself at home.
The anal hook is another object of pure intimidation. I have a hard time crafting up ways to use this in self bondage, since they are generally used in complex hogties, and other positions that require you to be in a bind – all of which are very difficult to do alone comfortably for any length of time.