My bike doesn't have water bottle mounts either, rather than hanging the u-lock from the top tube or because there isn't enough clearance between the tire and seat tube for the mounting bracket, I mounted it on the down tube. That way the angle helps keep the lock from falling while your pedaling from a major jolt. Granted I'd prefer it mounted on the seat tube, I'm just paranoid a good jolt will release the latch without me being aware of it and somehow another jolt and the lock works free and falls.