If you really must you can use a RAM clamp around the top screen bar but it may not be a good idea. I have tried using the top bar for my Garmin 2720 and it vibrates like crazy. Thought it was the RAM mount so bought Touratech's special adaptor to use with a Touratech stanav bracket. That was just as bad and buggered the paint on the bar.
I put the satnav back on the handlebars where it is rock solid and thought I would use the mount on the top bar for my bike-to-bike radio as that is much lighter. That shook about just as much but when I moved the mount to the bottom (original) bar it is rock solid.