I was thinking the same as Haird before I read his and Jerry's comments. I was riding to Daytona this past weekend and hit a stretch of road that put the 3pm sun perfectly at my back for about 20 miles. I as not a happy camper! I'm thinking "A million plus miles of test riding the Rushmore changes huh MOCO?!!!" Bullchit. I had to ride the whole stretch with my hand up to my face to shield the glare. Real safe I am sure....
Only thing I could think of would be some type of firm, non flexible, piece similar in shape to the screen with a suction cup or two on the back. Where you'd keep it when not in use is another story. I know a crappy method, but I would have used it had I had such a piece on that day. Did it again the next day in another area as well. PITA for sure.
On the subject of screen brightness, go into your settings and turn the "Automatic" off and set it manually to the brightest it will go. Helps during the daytime riding and you can readjust it at night or set it back to automatic.