I've used this approach for years to make darkroom doors light tight. If you don't have any rollfilm backing paper handy, use black construction paper.

Since these pictures were taken, I've switched to a different kind of tape. Originally, I used the black "gaffer's tape" found in most studios. The problem is that the glue in this tape dries out over time and leaves dust and a revolting mess on the door that has to be cleaned off.

I've switched to using painter's masking tape. This stuff has an easy release adhesive that should theoretically be easier to unstick and clean up when the time comes. We'll see. So far, the masking tape has shown no sign of drying up and leaving adhesive dust all over the place like the gaffer's tape did.