I've bought these, and they worked pretty well. They're just a lightweight or maybe light midweight cotton. Probably not a canvas. They'd be extremely easy to make. If you're interested in seeing some models for size (can I post this? I am not affiliated with the company or selling anything) then Reusablebags.com has some.

Here is a link that might be useful: Muslin produce bags sold online

My girlfriends and I had a sewing bee in early December to make similar bags. We made 3 different sizes using a mosquito type netting. Cut out using rotor and cutting mat. Very efficient. 3 sewing machines operating and after 7 hours we had made 55 bags! Zigzagged edges, then casing on top w/ drawstring. For presentation rolled 3 different sized bags and wrapped w/ raffia. A nice environmentally friendly gift plus we had lotsa fun in the making.