Starting with a 2 foot length of 3/32" wire cable, I crimped a ferrule onto each end using a vise. Now I can thread the cable through my helmet visor, tuck the ends under the handle of the left hard bag, and lock the handle down. You can easily make another cable for a passenger helmet, or a longer one to thread through the sleeve of a jacket to lock the jacket to the bike.

This cable takes about 3 minutes to make, and the parts cost me $1.88 from the local builders' supply store.

Here is a photo of the helmet security cable.