Or use a plastic bottle cage, like the Elite one that's pretty much the best you can get.
I have carbon cages, but if the flask/bottle is hard plastic or metal then it'll rattle whatever the cage is I'd have thought.
Use one of those neoprene, beer cooler sleeves perhaps?
No rattling and added insulation.
@TTM started
London Fixed Gear and Single-Speed is a community of predominantly fixed gear and single-speed cyclists in and around London, UK.
This site is supported almost exclusively by donations. Please consider donating a small amount regularly.
Or use a plastic bottle cage, like the Elite one that's pretty much the best you can get.