Contour of the difference between two images

For a university project I need to compare two images I have taken and find the differences between them.

To be precise I monitor a 3d printing process where I take a picture after each printed layer. Afterwards I need to find the outlines of the newly printed part. The pictures look like this (top left layer X, top right layer X+1)

I have managed to extract the layer differences with the structural similarity from scikit from this question. Resulting in the image in the middle

The recognized differences match the printed layer nearly 1:1 and seem to be a good starting point to draw the contours. However this is where I am currently stuck. I have tried several combinations of tresholding, blurring, findContours, sobel and canny operations but I am unable to produce an accurate outline of the newly printed layer.

The result I’m looking for should ideally look something the bottom image

The problem seems to be that the difference results in an highly “uneven” color distribution inside the desired contours. But when I start blurring the image enough to counteract this, I also lose the “correct” contours.

I have uploaded the images in the original file size and format here

Are there any operations that I haven’t tried yet/do not know about? Or is there a combination of operations that could help in my case?

Any help on how to solve this problem would be greatly appreciated!!

Does nobody have any idea what else I could try? I would really appreciate any pointers on what else I might be able to try!

I remember a previous thread on this same topic, probably by you. I remember giving some opinions there.