Clarinet flutter tongue on the upper register?

Nothing beat practice. Start fluttering at the pitch you can, and slowly work your way up to where you need to be. Don’t give up. You’ll get there. Don’t expect to have everything all at once, perfect the first time. But trust practice. If you practice enough, you’ll get what you need.

I agree with “practice” mentioned above. I remember having to practice different tonguing techniques to get them right in school. There was one girl who was a particularly good at tongue techniques (she now plays trumpet in the Baltimore Symphony Orchestra) and you would hear her practicing in the hall, in the bathroom, at lunch, everywhere. As far as the reed, get a couple to see which one makes it easier for you. A thinner reed may require less force to be manipulated, so it may be easier, while a thicker one may be needed for the resistance. I say go buy a few sizes (they’re still pretty cheap, right?) and talk to someone who is knowledgeable at the music store. Good luck!