Quick and dirty circle jig for my dewalt router in a quickly approaching speaker project. Uses a piece of 1/4-20 all thread, a 4 pack of couplers (only really need 2) and a piece of 3/16 brass rod. Works well enough for me. Feel free to use, change, whatever. My model isn't great so your mileage may vary.
Edit: I think I may not have scaled this to account for any print shrinkage, so you should check the size of your couplers and account for any shrinkage in what ever material you decide to use.