I have a multitude of these things. Current fave is the predecessor to the Caddy Sack, which was a zip up one by Lezyne (thankfully I bought a few), and it fits middle pocket perfectly. I also have the Rapha one, which goes in a side jersey pocket with phone, keys, cash etc.
I used to load the Lezyne sac with tubes, multitool, patches etc, which all fitted but was quite heavy. I have recently bought a couple of the new Lezyne Road Caddy tiny packs which strap under saddle and am finding that best option, as it just fits tube, tool, patches, chain link, leaving pocket bags for lighter stuff (I do take a second tube on longer rides)