Clarinet flutter tongue on the upper register?

I am in a pit orchestra for the West Side Story musical playing for a local high school, I am playing different instruments at times but at one point the score calls for a flutter tongue on a upper register A above staff. I can just barley manage flutter tongue with an open G, does anyone have a tip on how to flutter tongue the upper register or maybe even a certain size reed that would work best? How do you flutter tongue. (Somebody keeps saying that this question isn’t specific enough.)