(sorry, I wrote this post before I re-read and realized you wanted advice on a windscreen extension, not a recommendation on a windscreen)
I put a dark smoke zero gravity sport touring smoked windscreen on my 02 st4s - couple of inches taller than stock - doesn't get rid of all the wind but is a couple of inches taller than stock, looks good:
Sport Touring Windscreen - Zero Gravity
Here's some better prices:
ZGØutlet.com
I'm 6 feet tall and it "softens" the wind that hits my head and shoulders. I'm sure some of the other riders on the site have other recommendations - I guess it's a matter of how concerned you are about looks and whether you want to reduce the wind or try to eliminate it as much as possible.