I like the idea from @BlueThunder05 of his remix. But my PTC heater was 60^2 instead of 80^2. I wasn't able to change his FreeCAD design. So I exported it into fusion and cut 20mm sections in two dimensions (x&y) and 19mm in z. Additionally I add a hole for the cable of the fan. If you like to use the model be careful when inserting this cable because i forgot the inner-cable slot so you are in danger to cut the cable.
My heater fit perfectly.
The author remixed this model.
Original: 80x80 PTC Heater
New: 60x60 PTC Heater