Make a duct tape sheet just a little wider and long that the two sheets of foam.
Put the foam on the sticky side with a little space in the middle, at the top, and on either side. Make sure you get the entire thing stuck so press/rub it down.
Tape the other side of the foam. Make sure that the tape pieces are SHORTER than the foam on all sides. This doesn't need to be exact. But do make sure that they are facing in the opposite direction that the previous tape was.
Hem it by putting tape all around the edges.