I made these with a glue gun and recycled tops from single serve chocolate milk bottles (from fast food restaurants). We had six caps, so I sat down with the kids and a piece of paper and we planned our six designs. They each got to choose three. We ended up with square, pumpkin, smiley face, triangle, snowflake, and flower. Then I made the designs on the tops of the lids with the glue gun and we let them set for 24 hours. (I also did some designs on cardboard rolls, but those didn't work as well.)
We made some cloud dough the next day from flour and baby oil and the kids used the stamps. They really enjoyed using the stamps and they worked fairly well. (Cloud dough takes 5 minutes to make and is wonderful to play with. If you haven't tried it you definitely should.)