I looked at those at the start of the year when I was getting deafened by cold turbulent air on the long fast descents round here. I couldn't bring myself to pay for something like that, but did toy with the idea of making some.
90% of the noise seems to come from loose helmet straps. Somebody told me that and I tested it by pulling the straps flat against my cheeks on a fast downhill last week. Sure enough, the noise was drastically reduced. Someone else said that putting a twist in the straps can work. (So they effectively present a spiral profile to the air rather than a flat edge.)