OK, so here is the final part.
Got the installation kit i've ordered containing a mounting cage and two bezels for the new rounded AC slot and one for the HU since the space is larger than regular double din.
The plastic bar in the middle of the upper compartment needs to be removed. I saw it in line with the frame, meaning in the middle of the screw holes.
The cut does not need to be perfect as long as it is out of the way of the cage.
This seems to differ between models from what I've read, but my car has metal guides on either side that also needs to come of. The metal frame is released with two torx screws.
The metal guides are attached with rivets, that can be pinched of from the inside.
With the rivet heads gone the metal guides can be pryed of with a screwdriver.
Attached the locking mechanism from the kit I ordered on the HU. Need to experiment a bit with the length of the mounting to get the HU correctly aligned with the console.