I made a cafe lock with a 1.2 m length of wire from my sailing dinghy rigging when it was retired (not at all easy to cut through), two crimps to make loops at either end and an old shackle that needs a hex key to undo. It weighs about 70g, cost me nothing and unless someone has a small bolt cutter or a hex key of the right size, will prevent theft.
It isn't for leaving in a busy place but nipping into a shop for a few minutes. Unless the thief is tooled up it stops ( so far anyway) immediate wheeling my bike away. If a thief is tooled up he/she will get through anything I am likely to carry.
I can add a photo if anyone wants but the cat has plonked himself on my lap right now.