Its easy to make a simple stamp. I've just shown u how. If u are not confident to use a knife to carve on rubber or potatoes, just use scissors on foams. To hold the foam, u can mount it on wood, or any sturdy block from whatever material u can find. Old rubberstamp mounting can also be used depending on the size of your cut foams.
Draw a shape on foam, cut using scissors, attach foam to your wood or fiber mounting using mounting tape or double sided tape.
Stamp away! :)