This is an adjustable ellisograph that I made for my brother who is an artist.

It is slightly adjustable to change the ratio of the major and minor axes of the ellipse as well as adjustable to change the size of the ellipse.

Unfortunately, since the dovetail pieces have to be rather long to avoid twisting, the range of the axes ratio is rather small. A picture of the longest and fattest (with the position of the pen creating the smallest possible ellipse) ellipses that can be drawn with this tool are shown.

You will need a 10-32 tap, 4 10-32 nylon screws (or metal) and 3 10-32 nuts. I'd imagine 10-24 would work as well, or a 5mm tap of some sort (holes are 4.4mm or 11/64") .