u/Inevitable_Syrup_902

Hello! I'm preparing a piece to be performed by a small ensemble and there is a part in the piece that I want a free type of glissando inside of the piano's strings as I mentioned in the title, specifically with both hands flowing in opposite directions around the high-pitched range of the piano. Is there a specific way to notate that or is it going to be okay in a graphic notation of my choice? I was thinking of making a sketch that depicts the exact direction I want the pianist's hands to follow and explain what it means in detail on the performer's notes in the beginning of the score.