This is far from something restricted only to us hearing aid users, many on the pro side of the table do it also, but strictly speaking “open fit” and “instant fit” are not interchangeable terms. A powerdome is a “closed” fit that happens to be on the end of a slim tube. Open means there is a vent in it, and you can have an open mould (skeleton moulds) or closed domes. Many people do use the terms interchangeably, but they really aren’t.
The slimtube is an option, and also full custom moulds are an option on thin tubes. I am not sure exactly how it’s done because presumably you need to be able to pull the tubing out and change it every few months, but it’s certainly possible as I’ve seen it. If you want the best seal it might be worth jumping straight to getting moulds done, depending if you think you will tolerate them well. Then again, even moulds can be difficult to get right if you have the kind of ears that change shape very substantially when you move your mouth. I only had to smile and both earmoulds fell out!
Have you tried other sizes of domes? Might be as simple as having one which is bigger or smaller (as smaller might go deeper into your ear and thus stay put and seal better).