Thank you for your help, the main issue is that i need to have the corners quite accurate to avoid weird behavior when you drive the car on it.
The entire mesh is about 960 poly, splitting the mesh as one mesh per face i’ll end up with 128 meshes just for one corner + small and big sides, regarding the fact they are all concave i’ll have to make one mesh per face 
Is that the only way to go ? Any ideas on why the double sided option in UE4 only works for the vehicle but not the ball ( that would simplify a lot this process 
)?