Marshmallow: Use double-sided carpet tape (available at hardware stores) to cover the box with white felt.
Chocolate squares: Cover two sheets of cardboard with brown felt, using double-sided carpet tape. (The cardboard sheets pictured here are each 20 inches square. You can cut them from an existing box or buy sheets at an office-supplies store.)
Graham crackers: Cut two more sheets of cardboard (these are each 22 inches square) and poke holes in them with scissors.
Smush it all together: Using double-sided carpet tape, attach the graham cracker pieces to the chocolate pieces, then tape the chocolate to the front.