In one I got a little enthusiastic with the Xacto and cut through both layers in a couple of places, so on that one I cut another square from a third Foam Sheet and stuck that to the back to hold it together.
Will they last forever? probably not but they will be "rustic" which I love and quite inexpensive and fast to make. I am loving the ease of making a bunch quickly. 12 sheets of sticky back foam are about $8 on Amazon and I made about a dozen squares and a few long thin stamps from 2 sheets.