The only thing about the plain craft mesh is that it doesn't hold in the humidity the fish need for their labyrinth organ. But, you can take some saran wrap (the clear veggie wrap) and wrap it around the mesh, and just tape it down the edges. This will keep the humidity in, and provides you with an easy lid. :)
Another option is to go to a hardware store and get some acylic or plexiglass. Sometimes they will cut it for you, other times you may have to do it yourself, (you'll just need to buy a plastic cutting knife) and they can tell you how to do it. :) It's easy, and makes a nice clear top for your tanks. :)
Or, if you have some old picture frames hanging around, you can take the glass out of those and lay them across your tanks, works the same way.
Just make sure you leave a section open for air exchange along one side. :)