Screen selection is very much a suck it and see thing. Different bikes, screen design, mounting, rider height, helmet design, wind direction/strength and travelling speed all play a part that has one biker at 5'3" loving one screen while a 5'10 rider is suffering, or even vice versa!!
Some screens end in a flat edge that creates eddy currents, others have lips or even laminar lips that try to lessen those eddy's. Making a combination laminar lip if you have a flat edge screen may work, it's easy to find out.
You will need a piece of material, preferably perspex the same thickness as the screen, about 40mm deep and almost as wide as the top of the screen. A couple spacers clamped either end to hold it about 15 - 20mm off the surface and angled so it deflects the air up and over the edge of the screen and can break those eddy's and hopefully send the wind over your head. May or may not work, but the only other alternative is lower bars to lower your riding height, or a higher screen.
Send a pic if you can so we can see what sort of screen it is and better give advice as some are a lost cause.
Bill............................;-)
