Here's the trick. I made a compass jig for the router, just cut a square in mdf and mark the centre. Drill a hole in the centre to hold the centre pin of the compass and set your outside radius, then just set the depth and cut, then reset to the inside radius and cut again. Hey presto you got a circle! Then just cut one side and drill the holes for 6mm threaded bar. it takes a bit of setting up.
I will have a price list for N and 00 in the coming weeks for the standard radius.