There are two ways to accomplish this.
The least time consuming way is to do what we call Rollation, or roll-out layout.
The way to accomplish this is to layout your eccentric side-- 1/2 of the total.
As shown in the image below.
Then, at the location where the circle/ellipse ends, bend that at 90 degrees.
This can be done to handle a dual eccentric/ellipse fitting, as well as a circle/ellipse fitting. You can set your slope angles accordingly as well.
Once you make this, you'll have a piece of metal that will allow you to "roll out" the two piece stretch out you're asking for. As you're rolling out the piece, have a helper, or another mechanic scribing onto the flat for you. I.e., it's a little challenging to do it by one's self.
Then, make sure you add for your connection flanges/acme/ etc... to make up the fitting.
As for layout of the eccentric/ellipse itself, Bud has that posted in his library. Go there and you can find it.