I use my ski/snowboard retractable cable lock similar to this:
https://www.amazon.com/Pacsafe-Retra...2&sr=8-13&th=1
With the helmet on the seat, it keeps the seat cool when it's time to "mount up".
I also sometimes place my helmet in the right saddlebag. I didn't realize that the right has just a tad bit more room than the left, thanks to Lew's wife for pointing that out, LOL.