Ok, getting the outer shape is easy, you're making this way to hard on your self. Do the same steps as above, but when you take your photo, put a ruler in the pic, say a 20mm length. Then when you import the photo into SU, draw a 20mm line, and scale the photo to match that line. Bingo, 100% scaling. Then use the arc tool to make the arcs, tracing the picture. If it's a number of arcs with different radii, then make multiple arcs, but only half the item. Then copy/rotate the half arc into a bigger one.

As mentioned. 30 minutes.

But to be honest, I'm guessing every dimension is not critical. There are probably some parts that are open to the air, so design a new one to fit the attachment points, and the rest just needs to be close.