You can use about anything. If you have a local glass supply shop (or skills/tools to do it yourself) you can get glass cut. Or, you can use plexiglass or lexan from your local hardware store which is much easier to cut. You don't need either very thick, 1/8" should be sufficient, or you could go a little thicker if you'd feel better that way. Doesn't matter a whole lot in a small sump like that. If you were running a larger one with a lot of flow you'd want something thicker so it doesn't bow so much, though.