This super simple project is full of potential. Buy a sheet of the extra thick craft foam from your local craft store (you may be able to laminate the thin stuff together with a strong but flexible glue, but I haven't tried it). Next, carve out your design with a craft knife and use a small pair of scissors to smooth out any ragged edges.
Once you've cut out your shape in foam, glue it to a very thick piece of cardboard, a scrap of wood, or some masonite. I laminated some thick non-corrugated cardboard together to make my stamp and it's held up to being painted and washed off a couple of times without problems.
Once the stamp is done, go to town with your paint. I used mine to make some homemade gift wrap out of brown craft paper from the dollar store and some leftover craft acrylic paints. Cheap, quick, and totally customizeable!