Some outside pockets on bags are on the small side.
I recently decided to go back to using an older Baggalini bag with open pockets on the end and really happy to find a 13.5 oz. bottle the fits perfectly.
https://www.amazon.com/dp/B0D5HFCM51?ref=ppx_yo2ov_dt_b_fed_asin_title&th=1
Thanks for this suggestion Liz. I ordered the small 8 oz bottle and looking forward to not lugging around a heavy water bottle when traveling about.
Oh wow! This is cool Liz, I had never really thought of the option of getting a flat bottle. My current bottle doesnt fit and I've been lugging it around by hand way too long.
Flask.
I bought a plastic camping flask to fit in a small bag under a bicycle seat. I still have it. I don’t like it. Hard to clean. I prefer a disposable bottle or one with a wide opening for cleaning. Originally designed for booze. Booze is probably self-disinfecting.