Concentric meaning each curve is the same diameter? If so, than you're right since each curve will be a different diameter in order to keep the equal distance.
What do you mean by "All you need is a rectangle cut on a table saw"? Would the template be a piece of wood with a square cut in it? Or a square piece of wood?
Thanks for the washer tip!
Hi Aaron - let me back up a bit. I had to read clampman's post a couple of times myself to figure it out.
Basically there are two general types of templates, male and female. A male template, you run the router around the outside of, female template traces around the inside of the cutout. I was talking, and so were you, about a female template. A board with a hole in it.
Clampman is referring to using male templates and he is correct in that they would be much easier make and likely much easier to handle the corners with.
Assuming you will be using a 1/8" bit with a 3/8" bushing, all you would need would be a square template, 1/4" smaller than your proposed outline in both the X and Y directions. You can handle the corners by simply marking the corners radius you want and using a belt sander to round the corners to your mark.
If you do the outside one first, you can simply rip that one down to the correct size, do the corners again and you're good to go.
Just two things to watch: When you place the second template, it need to be placed EXACTLY right or your second pass will not be concentric. You should also place support strips around the work piece equal to the thickness of the template plus the workpiece to help support the router and prevent tipping. Actually, steps to avoid tipping should be taken irregardless of the type of template.
Hope this helps