Take aluminum foil and measure and cut to fit the dimensions of your screen top (you want roughly 3/4ths covered, then spray paint it black. Take electrical tape (make sure it's not stretched out when you apply it or it'll become a pain to deal with) and tape down the aluminum foil making sure there's no gaps.

I used this process and it's a pretty clean job. You want to make sure you dont cover the whole top though for airflow. You can also make cutouts as appropriate for your CHE/OTH