- Make the frame in two halves, four pieces per half, but don't drill the dowel holes in the end faces yet. Because you're not making a closed assembly, the joints will all be tight, assuming that they're doweled and clamped correctly.
- When the two halves are dry, use a straight edge to check across the end faces - they will either be slightly open or slightly closed.
- Take a piece of scrap plywood that's slightly wider than the half frame and cut two good, parallel faces on it - leave the fence in place after truing the second face. Clamp the half frame to the plywood so that the inner tips of the end faces (either at the outside or inside of the frame) just touch the face of the plywood and the triangular "error" is hanging over the edge of the plywood.
- Run the plywood with the half frame clamped to it through the saw and trim off the protruding ends. Repeat for the second half of the frame.
You now have two halves where the mating ends will fit up with zero gap - there may be a slight mis-match at the tips, doesn't matter because you'll trim that off when you cut the circle.. Drill the dowel holes in the mating ends and assemble. Now do your set-up and cut the circle. Works every time.