You could use poster board and glue the suction cups in place.I want to put something on my new tank but it's full and cycling...could I paint it now as long as I don't get paint in the tank (not sure how I would do that anyway)...
Black construction paper is a great idea...only issue for me there is I use suction cups on the back outside of my tank to better secure my intake/outtake hoses and the air line. I'd have to get it all on and cut out sections I guess to get the suction cups to work.