Zephal used to make plastic straps that tightened with the waterbottle screw. Worked very well for me on my bikes without seat tube bosses. Both bikes needed repaints so I had bosses installed at the time so its been a few years since I needed them. I'd try their website and see it they are still offered. They were very kind to paint and didn't move unless you hit the cage. Big step better than the velcro ones but putting them on isn't something you do on impulse before a hot ride.