Low Relief Cardboard
Create an image with a minimum of 4 different layers of cardboard. Work any size you like but keep in mind it is difficult to cut cardboard into small pieces. You can stack pieces of cardboard without permanently attaching them. If you would like your piece to be permanent you will need tape or glue, but this is not required. You can use colored cardboard, marker, or paint but the use of color is completely optional.
Examples taken from various sources
Low Relief Student Examples