c# - Detect rectangle in picture with various perspective and lighting

I´ve got a problem. I am taking pictures of a common solar module with a camera flash. I need to detect the frame of the module to cut out the module and undistort it (I only need all of the cell area (dark area inside the frame)).

sample image - direct flash --> problems with big reflection ( I think i can reduce it with a good diffusor)

sample image - flash from angle

Anybody have some recommendation for a robust method to detect the frame? I need something to work with various image angles and lighting.

processed sample image 2

The last picture is processed. I blured the image, grayscaled, inverted. After that I thresholded the image and tried to detect contours (Got some Problems with the shadow on the bottom of the image)

Thanks for your time.
Chris