Detect elliptic shape in image

I habe an rather complex image to analyse. Here is a quick thumbnail of the image (I attached the full sized image (Image_Full):

Goal is to measure the elliptic shaped hole (radii and area) in the center of the image. I tried the "Ellipse Detection Using 1D Hough Transform" from the File Exchange but I can't finde any suitable settings to get the parameters for the ellipse. When I got this radii and area should be easy.

I also tried variouse image filters to get a better edge image. I obtained the "best" result (in my opinion) by applying Otsu-Methode for image binarisation then using two morphological filters. First I used a closing operation with a round shaped structuring element (3px radius) and then I used a opening operation with a round shaped structuring element (5px radius). After that I used canny-operator to obtain the final edge image. I attached the result in the attachments.

Question is now whats the best method to get the elliptic shape? Maybe you can guide me into the right direction...