OP here. I bought this
Ergodyne Chill-Its 6660 Hard Hat Brim $10 from ebay.
It works VERY well for me-
- GREAT shade/coverage on sides, ears, face, + back
- front is 2-1/2" (6cm) = long enuf to shade my face, short enuf to not obstruct visibility or catch much wind
- the tail in the back is small enuf to not obstruct the helmet-mount mirror view, and hung from the brim = keeps it off my neck = good airflow + not excessively hot
- stable in straight airflow, including longer downhills at 35+ mph
- minimal buffeting when trucks passing or crosswind
To get it on/around the helmet I cut about 1" out of the front of the brim. With that done it went on easily. I also melted 2 holes in the front brim + wired it to the helmet, to prevent it from blowing/sliding down. There's a piece that fits under the front of the helmet which prevents it from going up. The back fits tight into the back of the helmet, and has 2 velcro straps which loop around the helmet straps nicely.
YMMV.