I looked at a lot of those "hard-mount" possibilities, but I found this instead and have been quite happy with it. It's called Helmet-Lok and I found mine on eBay. I got the T-bar model that goes through the rings, but there is also the option of a small cable instead of the T-bar. I attached one pic of the packaged product and one pic of my Helmet-Lok on duty. I just rest the helmet over the mirror and lock it to the cross-bar; two helmets? use both mirrors and two locks. Of the available similar products, I selected this one because it is a four-tumbler lock hopefully making it a little harder to sort out and steal my bucket.
I keep a small cabled pistol lock with me tucked under the seat, in a saddlebag, etc. and use it as a helmet lock in the rare occasion I leave it with the bike.
These locks are usually coated to prevent scratching stuff, sturdy and have tiny keys that are easily kept on a keyring.
Don't have one? You can buy one cheaply at WalMart or a gun shop.