A simple bicycle chain lock bought in the hardware store next door. It’s length is 1,20 meter and I twist it around the pipe between saddle and luggage rack.
When needed I thread the lock trough the back wheel, twist it around something very good rooted and then thread it through the gearwheel before I lock the lock.
I’m careless and let them alone for hours when I dive into the parallel universes of my favourite pub and the book store. (As I don’t know as which gender my Strida considers itself I call it politely they/them. Actually I think they are nonbinary. Because triangular entities might prefer it that way.)