>Do you ever make hypnosis files purely for yourself? For example, something with basic background binaurals and a TTS voice.
Sorta. I once prototyped a very simple "video game" which was a program designed to keep a sub's attention focused while running them through a trance. It used binurals that adjusted to the game state and TTS as a placeholder voice.
>Or is it too hard to trance to something you yourself scripted?
No, it's about the same process, but it's not as exciting in terms of novelty or sexual appeal.
>Does hypnosis need the element of surprise, or recurring amnesia of the file contents?
Neither of those things are required for it. The state of hypnosis is no different from meditation.