Needing to play with my new toy (a 24" omnijig), but not actually needing any dovetails at the moment; I found that a 15/64" straight bit with a 7/16" guide cuts a perfect box joint with the standard dovetail template. It is good for up to 1/2" wood.
Have I discovered this, or have people been doing it for centuries? Okay, it is not a big deal, but it is cheaper than buying the box joint template.