Edit 2020-02-19: Thanks to p4nd4x for his remix! He did a splitted version so you can print in two parts without support. Furthermore he created a printable slider so you need only two hex M5 screws instead of a part of flat aluminum or similar.

Since all available designs for Makita circle jigs on Thingiverse didn't either look stable enough or needed parts currently not easily available I designed a new one.
With simplicity and printability in mind I designed this one.
Unfortunately this thing is unprintable without support but at least it does not waste to much filament and should not need much post-processing.
Everything you need to complete is a piece of whatever (I used flat alumininum) with a profile of 20x5mm (max 6mm), a ~5mm thread on one side (slot is 5.5mm) and something pin-like on the other.
If you keep a distance between pin and threaded rod of ~45mm you can create circles of ~8mm radius up to 200mm without a gap in between, depending on how much material you keep around the center-pin.
You should keep some material (>10 mm) beyond the threaded rod to give the slider more stability when set to maximum radius.
To avoid scratches on the surface make sure the threaded rod does not protrude. I used a M5x20 sunk head screw glued in with epoxy.

The knurled grip for a M5 nut is available as my Thing 3730440.

The design was based on Thing 2611305 and tested on a Katsu clone so it should fit the Makita routers as well as the clones.