Put an index of symbols at the beginning of the score describing special events like you mentioned and use that symbols throughout the piece.
It is best to come up with a new symbol or an obvious modification of the regular fermata sign. You can also define the regular fermata sign to refer holding until the sound fades. But this may confuse some players or they may forget the meaning and play traditionally because of habits.
Providing an index is a common practice in modern composition as many pieces introduce different ways to interpret the score rather than rely on musical traditions. Consider the increase of use and variety of signs over time since Baroque until today.
I also suggest to calculate yourself and write in standart notation corresponding to the the exact length you need, as this will be quite accurate and forcing the player to play directly what you intended. This is also a common practice in modern notation where the composer asks for more precise interpretation and don't want to leave things to chance.
Using terms referring the passages rather than single notes may result in unintended ritardando's and diminuendo's or some other unexpected effects spread to the whole passage rather than the note to be held until it fades.