In case you haven't spotted it yet -- Giro Venti? It's like a Giro Indicator, but it's bigger, listed with a 25.5" circumference. It would be worth a try.

http://www.giro.com/us_en/venti.html
http://www.bicyclebuys.com/helmets/HelmetMTB/GIVE0PART
http://revolutioncycles.com/product/...ti-61619-1.htm