-Bicycle inner tube 20" or 16"
-1/4" Metal chain 7' length
Cut the inner tube and slide it over your chain, leave some chain sticking out on both sides so you can lock it.
I doubled my chain because it was made out of a brittle metal so if ever someone had to cut it, he would have to cut through two layers of chain.
Make sure to use used inner tube and if possible try to use a recycled chain.
I use this chain and a lock to secure my bike when i run errands. I lock by front wheel to my frame, it can be easily cut if someone had the time/patience. But i rarely leave my bike unattended in sketchy locations.