I don't know whether you'd count mine as a "u-lock". It is one, but has an ultra-short shackle and can only be used with a chain or cable. It lives hanging from the left-side seat rail when not working.
I used to have a conventional Krypto long-shackle lock, the kind they later found could be opened with a ballpoint pen cap or similar. That one lived in a dedicated holder fastened to the seat tube when not working. I don't have frame-mounted water bottles, so it didn't interfere with anything.