I took Leah Day's class last spring at MQX in New Hampshire. In preparation for the class, she sent us a supply list where she had us prepare 20 12-inch sandwich squares in solid colors... they went together rather quickly. We only used about a third of them in her class so I still have a few that I practice on here and there... you can change thread color and stitch over previous designs.
I plan to make another stack in the near future... they are quite handy, especially if you are learning a new FMQ technique or need to check your tension before starting a project.