Simple Seam Wheel by Jen Kingwell
Place the Simple Seam Wheel next to a finished-size template, put a sharp pencil in the hole, run the wheel around the template to add a 1/4" seam allowance.
And here's a suggestion... You might want to order 2 of these. They are inexpensive and small. I sometimes lose little bitty things. Is that just me, or do you do that too?