I started down the same path and then I decided to do the "Chinese Blueprint" method: I've bought about 3-4 different types of sandals and flip-flops and just tore them apart. Pulled apart stitching, cut with an exacto kife through the glue, reverse-engineered the straps, etc...
Don't over-think this. They're flipflops. You can go to thriftstores and pick some up for 3 bucks and have at it. Plus, once you've taken one apart that really fit you, there's your template.