DesmoTimes sells a plastic one for $24. I had one of those on my ST4s for about four years and then transferred it to my ST3s. The nice thing about this one is that you don't have to relocate the turn signals when you put the luggage brackets on.
It finally cracked and broke after about eight years of use, but I definitely got my money's worth. I used it as a pattern and built a similar one out of sheet aluminum.