Remix of Chrismettal's remix. 
First contact with FreeCAD so my model is not perfect. I tried to remove the opening for the square nuts and increased the diameter of the screw hole to 4mm to fit a standard M3 threaded insert. I used standard M3x10 screws but M3x8 or even M3x6 should suffice, too.

Please check the correct alignment of the adapter in relation to the sides. The screw has to go in the hole directly besides the small slit in the side panel. Otherwise the holes won't line up properly.