Hi Geo,
no experience here, but Strida wisdom says:
“Never leave her alone and you do not need a lock!”
Maybe you noticed that old thread;
Handcuffs as locking device
Nowadays it’s easy to see what it takes to open any lock, just check these few examples;
https://www.youtube.com/watch?v=QGCgHiMYBpc
https://www.youtube.com/watch?v=MhwvRQpYquU
Already a while ago I was thinking about a lock, too - here’s what I found (halfways) acceptable:
This Kryptonite lock is pretty heavy and there is not much room left to secure the bike onto a pole,
if that’s required it would be possible to use an optional steel rope loop,
but it doesn’t interfere neither with folding nor pedaling.
Cheers,
Chris