Car holder for Märklin 4612 car transporter

My Märklin 4612 car transporter has the disadvantage, that the cars on the lower deck keep rolling around. I constructed an adapter, that fits in the car transporter without gluing and fixes the wheels of the cars in two slots, which keeps them from rolling around.
Picture 1 shows two holders with cars inside the Märklin 4612 car transporter (tilted to show that the cars stay in place) and Picture 2 shows the holder with the two slots where the wheels of the car will be placed.