The clamp with the torx bolt is a great and simple idea. But if like me you share your bike with your wife its needs to stay adjustable. A low level of protection could be a simple small cable looped around the frame of the seat itself (and around the bike frame) with a small combo lock.  Or a better cable looped around whatever locking method you use to keep someone from stealing your entire bike....   

I'm considering buying a brake disc lock (common on heavier motorcycles, not as effective on a 70 lb bike of course), and then you could run a thin cable thru the disc clamp.  I'm considering this Kryptonite K004776 disc lock because it has a loud alarm built in, but I'm not sure how much you have to move the lock in order for it to activate - would just picking the bike up be enough?  But if it is just to use while in a store (and not overnight outside your home), it might be enough.  https://amzn.to/3sFoaTZ