You need some fairly flexible plastic, although straight up flex-filament will not work.
I used PETG, with some good results, but I imagine that PLA would work fine as well. Alternatively, you can download the source FreeCAD file, and change the thickness of the buttons separator.
Print with the button faces down on the printbed, and at a layer height of max 0.1 mm (I used 0.08 mm).
I know this might be quite a niche model, but figured it might help someone else needing to replace their Kia car key membrane. And this project just happend to be around the time of the Replacement parts contest.
The author marked this model as their own original creation.