With a good tight cage it should stay put.
If you can orient it towards the front of your bike so that the bottle hits the stem on the way out, that can prevent accidental ejection. Makes it a little harder to get out…but it can act more as storage instead of constant use. So for instance use the one on the downtube 100% first, then swap with the top tube, so you only remove/replace it once.
Also, getting three 1 liter bottles instead of the 750ml adds some capacity.
