They are finally installed (sort off).
I left the rubbers on the car for a few days, without the metal seats installed.
After this, I was able to push the seats on with the help of some grease.
Like you can see in the picture, the rubber is a bit deformed, maybe it will settle over time.
Not the correct way I guess, but it works. Maybe I'll look at it later, but now it's time for the engine and gearbox.