For now, I'm using thin, hard-copy books. I don't think that's good sice the air for them wont be as good, but I take it off when I come home and I am in the room.
I use the plastic mesh that you can get at your local craft store... the same plastic mesh stuff that I use for the DIY tank dividers. Its nice because you can cut it to size.
I used a bowl for a couple of weeks and I put a mesh bag over the top and tied it on with ribbon, it looked awful and the fish looked distorted so she has a tank now! I would definitely cover the bowl with anything for the time being just to be safe!