Zip ties.
Are you using an aero bottle or a normal bottle? If a normal one, there a few cages that have slots designed specifically for zip ties like the TriRig Kappa. That's the setup I used here. (note I'm now sponsored by TriRig but wasn't when I had this setup)

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Benjamin Deal - Professional - Instagram - TriRig - Lodi Cyclery
Deals on Wheels - Results, schedule, videos, sponsors

Thanks! I'm using Crono CX aero bottle just like TO.

Have you used that bottle/cage yet? That won't be an easy in/out combo for BTS set up.

I mounted it on the down tube and it works really well. I put concentrated Tailwind for the long races. I cut the bottom of it and use it as a tool box as well and that’s why I want to mount it behind the saddle.

Short ride/race: BTA bottle for hydration and Crono bottle as a tool box on down tube.

Long ride/race: BTA + concentrated Tailwind on downtube and Crono bottle as a tool box behind the saddle.

As a tool box that you don’t need to pull in/out during a ride, ok, that makes more sense. I just wouldn’t want to be reaching for that bottle regularly while riding.

Exactly. I use tubular so I need a long storage unit to carry Vittoria Pitstop..

I use the Xlab single bottle system, with a Profile Design adapter for two CO2 cartridges

Picture attached