I’ve got a tiny 0.3l one from decathlon which clips to the saddle rails, no tools required so easy to move it between bikes. Just holds a CO2, lightweight tube, patches and multi tool.
Got a couple of different sized airwave ones from CRC as well (about 0.4 and 0.7l I think), they clip on with a buckle style fitting which is screwed to the rails. Use them on the MTB as they hold bigger tubes etc, brake pads, chain links, and a few other things.
Usefull having one on each bike with the correct tubes, chain links, brake pads etc. and saves back pockets for gillets, arm warmers, food, etc, and saves having to search through the garrage for the correct stuff to fill the camelpack with (or carying everythign but the kitchen sink every ride).
Whatever you get (apart from the tiny 0.3l one), put everything in an old sock, and the innertube away from anything sharp, otherwise the rattling will drive you round the bend and you’ll have holes in the tube before you even fit it.