When I was a teenager (aka, the Stone Age), riders carried spare tubular tires wrapped in newspaper or old socks. Now it’s considered cool to strap or tape a spare tube to your bike’s frame or seatpost. Whether it’s actually cool may be open for debate, but exposing spare tubes to the elements (or the contents of your seat pack) isn’t the smartest choice.
Storing inner tubes in a plastic sandwich bags is perfectly acceptable (and better than nothing), but a section of a Tyvek shipping envelope is even better. Compared to a plastic bag, Tyvek not only provides more protection from abrasion, the lightweight material’s superior strength makes it ideal for booting sidewall cuts (something I’ve done more than once). While the Tyvek boot may not be a permanent fix, it’s usually strong enough to get you safely back home.