What is important in a good quality snake bag?

I ordered one from a site that advertised "muslin" snake bags, but instead recieved a heavy canvas snake bag.

I've also seen a lot of people talk about just using pillow cases when a snake bag would have been useful.

Does it really matter?

My feeling is a heavy canvas is probably appropriate for most cases as it is rough enough to provide enough abrasive to help with sheeding, it is absorbant enough to keep moist for a long time, and it is strong enough to resist tearing.

What else am I missing?