What sort of cover should I use?

[gregzz4]

Why don't you just go out and spend the few bucks on a sheet of eggcrate and try it before you take all the time, money, and effort you'll spend on making a mesh cover ?

With your small tank, one sheet of eggcrate/light diffusor will cover it for, what, >$20 ?

And you'll have a bunch left over for other stuff

Give it a try
I love mine