After much research, it was impossible to obtain the desired result.
I have to make a bathtub-type tray, nothing too bad, except that, the surface serving as an edge must be inclined by 15° on the entire perimeter, the bottom has a notch that should help with positioning.
I think I've tried everything, the problem is that the notch at the bottom doesn't keep the initial shape and the "tray" is higher at the corners, it seems that I need to create profiles in the corners to remedy this problem, but that doesn't prevent it from getting stuck everywhere.
I was able to solve my problem very very early this morning, it annoyed me a little not to find the walkthrough, and as I am, sorry, more Breton than French, I'm narrow-minded.
So I actually took my two sketches representing the center and the rectangle, with the Extruded Surface function, then I used Filled Surface and finally Thicken.
For the "finishing, removal of material and fillet to obtain totally satisfactory as shown in the following screenshots.
And here is the method used for those who might be interested, for the others I attach the file.