Jake, you really like the craftsman stuff, that's cool.
Better yet, just get a large piece of styrofoam, used to pack some item from work, or hospital. Cut it to shape using a serated kitchen knife to fit your rear crossbar, and a groove for your mast. Lash it down with any old piece of line, and for get about it! It will take you about 5 minutes to make, and it's free. That's what I've done for years!