Any kind of plastic or nylon mesh/screen will work to cover the overflows. The size of the mesh grid will need to be determined by what size fish you have.

As far as the replacement clown fish goes, were the two you had paired up? If they were paired up and sharing an anemone or another type of coral, I would suggest getting the same kind to replace it. The remaining clown fish may be territorial in the beginning towards a new clown. Adding a different species that is already known to be aggressive on their own probably wouldn't help the situation. I had a pair of true percs and one passed away. It was the female, the large of the two. It took me awhile to find one of similar size and the original one chased off the new one for several days before accepting it.