Not really a response to anything, but you can make a jig for an ellipse
by using two grooves at right angles to each other to guide a stick having
two guide rods sticking down into the grooves and a pencil at one end.
Put the intersection of the grooves at the center of the proposed
ellipse and just crank the stick around. Max length between the rods is
limited to the length of the smaller of the grooves but max size of the
ellipse is limited only by your ability to control the pencil end.

Bill