I made a short lock/cable combo that I just use to lock around the top tube to keep someone from walking off with the bike when I'm at the store.

Building supply stores sell uncoiled, plastic covered cable of different diameters. I bought a legnth of fairly thin cable, made loops in the ends with cable fasteners and I use a small combination lock to secure. Very light, very compact, fits easily in the shoulder bag. Cost of the cable, less than $4.