What is the right material to use for these? My daughter wants to crochet me something for Christmas and I was thinking she could make a bunch of these protectors in colors to match my basses, but I want to make sure she gets the right kind of yarn.
If she uses yarn made of 100% cotton or wool or linen (or mixtures thereof) you should be 101% safe. However, most yarns with synthetic parts should also be pretty safe for your bass. Problems with stands and an instruments finish are mostly related to nitro finishes and synthetic padding.
Anything that is safe to use as a piece of clothing for a human should be safe for your bass' finish.
I hope you love it as much as I love mine!