This is my preferred method for odd shaped drivers, it's straightforward and produces very good results.
Simply measure from the outside edge of the router bit to the outside edge of the template guide, and let's just assume this is 1/8". Now find a washer, (or anything round with a hole in the middle), that measures 1/8" from the inside edge to the outside edge. Put the driver onto the template material, put the washer up to the driver frame, put your pencil in the washer hole and roll in around the driver. Once cut, the template will produce exactly the correct cut-out for the driver. Use a washer that is a tiny bit wider, rather than smaller, so that the cut-out isn't a too tight.