I saw this design on the Woodwright's show (that one was brass and wood). I needed this for a large frame so I replicated that design I saw on the show. I reused the same holders for the 1/2 in. X 1/2 in. arm that I had designed in my compass (VEC) design. Mark 90 degrees lines for the middle for the ellipse and measure/mark the height and width and slide the pencil to the mark and then tighten the brace. Move 90 degrees and move and tighten the slide to the appropriate length. I used tape on the bottom to hold this during drawing. I added small notches in the ends to make alignment with the lines easier. This design uses 11mm X 4mm ball bearings with a 5mm ID hole.
YouTube video in action; https://www.youtube.com/watch?v=fBQBb3EDSPw

MakerGear

MakerGear M2

No

No

Med

50%

You may need to clean up the edges just to eliminate any roughness that interferes with smooth sliding.