i recently bought one of these,
https://www.ebay.com/itm/HIPLOK-Z-LOK-COMBO-COMBINATION-SECURITY-ARMORED-ZIP-TIE-BIKE-LOCK-BRAND-NEW/173783213545?ssPageName=STRK%3AMEBIDX%3AIT&var=472456412125&_trksid=p2060353.m2749.l2649
they also make one that has a small key for 1/2 the price of the combo lock.
i like the look of it because it is easily stored by wrapping it around my under seat bag without scratching it if i use it. it's not a kryptonite U lock for overnight security. it a small out of the way lock to it prevent a crime of opportunity like the OP is describing. like where you leave a bike unattended for a min or 2 while you take a pee, or stop at the store to buy a water because you ran out on a solo ride.
the same company makes a small cable lock about the size of a pack of smokes that has a built in light on it and a clip to lightly secure it. slip it in your rear jersey pocket, turn on the light for extra visibility so it has a addition use besides light security for your bike.