I used a heavy black cloth when I had mine. I attached sticky-back velcro to the top edge and added a few sheet metal screws to help hold it in place, then I had my wife sew the other part of the velcro to the top edges of the pieces of cloth. Then it was a matter of simply attaching the cloth to the frame. I used 2 pieces of cloth so it was split in the center for easier access under the tank.
Why do I love fish? Because every time I stare into the little world I have created for them, every problem I have just floats away....