Sewing is not my strong suit, but I find circles particularly difficult to make. In prior attempts, I can not get the material to comply with anything other than a very rough, exposed seam allowance mess. Perhaps my inexperience showing through.
By contrast, I can sew a decent french felled seam in a straight line. Biggest challenge on my test pieces for the ridgeline slot hood come at the ends. I ended up bar tacking all the way through the four layers (eight including fold overs) to get it to look decent.
I'd like to examine Ron's pattern, but I already feel a little guilty for stealing the basic idea from his photos.