Thanks for all the ideas so far.

I'm really looking for simplicity here, and durability, and a low dork factor.

Having noticed that holding a finger up in front of my ear cuts the noise dramatically, I cut and shaped a couple of finger-sized pieces of minicell and taped them to the forward strap of my helmet. Pretty low-key, appearance-wise, but I don't know if the placement will be optimal.

Didn't get a chance to try them today, but I'll try them tomorrow and post again.