Hi! I was getting fed up by my 40 year old well used seat foam in my ke35, so I decided to make myself a new one with better support. I was looking for the right type of foam that's hard enough to give some more support during cornering and longer rides. The best foam I found for the task was actually sound absorbing foam!
I used contact adhesive to glue the foam layers together,and you can use a grinder to shape the foam. It's kind of tricky but it works! You can also use a wood file for narrow areas.
Finally I attached some "regular" soft foam for the cushioning feel. The blue ones is bought from a supermarket as they are made for drying the plates after doing the dishes.
Now the new seat foam is inserted into the car seat and it feels great! The lower back has now more support than the sagging old foam ever could provide the last decades. The car's handling also feels better (but I know it's not true ;-)) because of the new foam's supportive sides.