It's easy to build your own cheap circle jig for your router.
But ALWAYS remember: A router is a dangerous tool. Never swap milling cutter while the router is connected to the wall socket. And always wear safety glasses and ear protection!
Step 1: Go Get Your Parallel Support
As most routers are delivered with a parallel support, it's easy do build your own circle jig.
Luckily the parallel support was bolted in my case (have a look at yours - good luck!). If yours isn't bolted, just go and get two rods and go on with this instruction ;)
Step 2: Make a Bar and Add a Pivot
As my parallel support was bolted, I just had to remove the screws and replace the stop bar with my own cheap and easy circle jig.
You can make the circle jig with just a wooden bar and a bolt, or, if you're more into metal works like me, you can use a metal block and a bolt or just a piece of flat steel and a bolt (that's my preffered solution and you can see in the pictures above).
A big disadvantage of this solution is you can't create really small circles, as the bar of the circle jig cannot be closer to the milling cutter than the outline of your router.
The smallest radius you can mill that way is the distance between yout bolt and the cutter.
Step 3: Make a Circle Jig for Smaller Radius
Too bad I had to mill a 6" radius (for a loudspeaker cabinet) and my previous jig was not suiteable for such sizes.
To create smaller circles I just used some sheet metal.
If you're able to chamfer your sheet metal and bend it, you can make a nice looking circle jig as seen on my pictures. But no worries, if you don't have the tools for it, just buy some sheet metal in the right size, drill 3 holes (two for mounting, one as pivot point) and screw or glue it to your wooden/steel bar.
Step 4: Make a Pivot
As pivot point I just wondered which parts would be really cheap and easy to get.
I ended up using a threated rived and a counter sunk bolt (have a look at the pictures if you don't know what this is).
Step 5: Here It Is - Our DIY Circle Jig
With my new cheap'n'easy circle jig I'm able to cut circles as small as I want - I even can reverse it to enlarge the radius ;)
Enjoy your new tool!