Here is my way, it is not a cheap alternative. I carry them in my saddle bags al the time. It takes me a only 3 minutes tops to lock my helmets to my bike. I have 2 pair of the Medium Master Lock Street Cuffs (They around $99.00 each and are rubber coated). I hook 1 end to the D rings on the helmets and the other end, I connect to my forks. Then I rest the helmets on the mirrors and handle bars. This works great for me. I have tried many other alternatives but found out the hard way at bike week last year when it rained. My helmets were soaking wet. This way the inside stays dry and clean. I have gotten my first HD 100th anniversary full face(had to buy another) stolen off my bike before (at an HD ralley) and I trust this method to protect my 2 $700.00 helmets. I have also used the cuffs to connect my bike to a solid and unremoveable structure. I had a friend get 2003 Fat Boy stolen at a local bike fest (Y-BOR) on the main drag.
I use a small cable lock which can be used with one or two helmets.
You can attach anywhere it suits you on the bike.
I usually use the left front frame rail or fork leg (obviously avoiding the hot exhaust side)
Thanks for the great ideas...I made one tonight that's very similar to yours. I used a car cover cable lock which had one end that was looped and one end had a flat washer-type connecter. First, I shortened the cable because it was pretty long. I connected the washer-type end with an existing bolt and left the other end with the loop free. I use it by pulling it out the left side, underneath the seat, looping it through the helmet, and then back under the seat. The loop of the cable then goes around the seat post lock when it shuts. I like this because I don't have to carry any other locks or keys, and it winds up around the gas cap (or under the front of the black rubber guard).
Also, you could use one of the flat, metal helmet lock extenders that they sell if it had a hole in the end large enough for the seat lock post to fit through it....i.e. slide it through the D-Ring on the helmet and then lock it right to the seat post lock. I'd be a bit concerned about the heat from the pipes on that side though.