Seat for 2 years old child

When I had a booster seat in mine I put a pillow under the base of the booster seat so it would sit flat. Not sure if that helps. Also I think you need a tether bolt to anchor the top of the child seat to the rear shelf.

 

The front seat is a lot easier to manage, and it is allowed to place a 9-18kg child in the front seat if the rear seats are not suitable or if you already have two 10 years old kids in the back. Not sure about the law for the rest of Europe, so I would not cross the border without asking the police officers first, but at least in France that's permitted.

As a last resort you can remove the rear pillow and have some foam cut to size in order to fill the hole between the two rear seats. Also the pillow size of the earlier models is less massive than 400gen2 and 412, so depending on which cars you are considering maybe that and old pillow could do the trick?

 

I used a Maxi-Cosi in the backseat of my Maserati GranSport which was a fairly small area.
https://www.maxicosi.com/us-en/