A couple of years ago I was in urgent need for a router circle jig. This was just after I got the Festool OF1010 router. First I thought about just modifying any of the non-adjustable circle jigs for my old router but then came up with a different approach.
Instead of just screwing a piece of MDF to the router base and then drill holes for a pin at different distances from the router center, I mounted the piece of MDF to the Festool FS-OF 1000 Guide Rail Adapter (488752) using four M4 screws. By doing so, the router can be adjusted relative the MDF plate and locked in place wherever desired. I marked the center line of the router bit and drilled two different holes to mount the center pin for the jig. The slot for the router bit was done with a 10mm router bit. A safe way to route the slot is to attach the MDF plate to a scrap piece of MDF with double adhesive tape. Then just plunge the router and slide it on the metal rods. Voilà! Router circle jig complete!
As an extra feature, the Festool FE-FS/OF 1000 Fine Adjuster (488754) can be used in combination with the jig. Unfortunately Festool suffered from temporary brain damage when designing this product — it protrudes under the route base! Just route a dado in the MDF plate so that the fine adjuster can be moved freely on the rods.