On this forum, there occasionally are comments on the merits of carrying a chain whip for removing the cassette on tour. Yesterday I fabricated a new chain whip idea that is a lot lighter (mine weighs 42 grams) and portable than the commercial whips. You still need the tool and wrench to unthread the cassette lock ring, so it is not the perfect solution.
You want to use a cord that is not very stretchy, nylon stretches more than some of the other synthetics like polyester. Some shops tighten the lock ring on pretty tight, I tighten it only a bit more than enough to make sure that it stays on. It however is best to know if this will work on your bike at home (lock ring loose enough) before you really need it.
Yeah there are some specialized tools that allow you to unthread the cassette lock ring without a chain whip, but I have never actually seen one. I am sure others will post the names of those as a followup.