What about one of these gauges? They are absolutely fantastic to see if people are actually using a 1/4 inch seam... And you can use them to do a fun little seam test at the start of your retreat. You give everyone 3 pieces of 1.5" x 2.5" scraps of fabric. You sew two pieces together along the long edge, and if you are using the correct 1/4" seam, the third piece if fabric will fit along the top exactly. Once sewn together, the square will measure 2.5" x 3.5".