On my old bike, I would zip tie the cage to my seat and run a smaller zip tie around the front of the cage to "snug" it. With the bottle in the picture, it fit like a glove, in fact you could almost feel the bottle snap into place once you dropped it back in the cage. The angle was perfect, never launched a bottle. I just bought an IA10 and I'm planning to do something similar, though I want to try to DIY my way to 2 bottles instead of 1.

I got the idea based on Rinny's setup, she ties a cage to her seat as well.

Maybe grab some zip ties and see what you can come up with? Especially since it sounds like you run a single bottle BTS, should be easy. I used a 7 dollar cage from my LBS and drilled out small holes in the cage where needed to run the ties.

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Regards,
J. Smith