How Can I Change Object Collision Shape?

Hey guys i am trying to change this object collision shape, but it says "Shape most be convex" is it possible to change it perfectly like in the shape?

 

All "points" must be external not internal .

 

If you really need that specific collision shape you should divide your object by 2 or 4 like this:

Each object will have its own square (4-points) collision shape.

 

How can i do this in order to be perfectly collisioned?

 

Build one object from few others.