Semipro's suggestions are pretty good, using a template gives you some nice control. You do have to make sure you're going the right direction so the bit is always pushing out as it rotates, otherwise it will likely jump out of its line at some point, most likely as you've just about finished the job. The other options would be to make a second pattern just the width of the cutter smaller all round so you're tracking in what is effectively a groove. You'll have to use double sided tape to hold it in place and place it carefully using the bit as a spacer, but I probably would stick with the outside template and just be careful to route moving counter clockwise, and I'd almost certainly do it with a small, trim router, in one pass.
The more I do, the less I accomplish.