I remixed Arduino Mega Case because in all available models we had to mount arduino board to bottom case by screws and close top and bottom case by another screws.
In my version there are needed only 4 screw. Arduino board will be holded by case like on the picture.
Not tested, I am waiting for parts to extend my print dimensions to print this project.
I uploaded *.stl file and source file from DesignSpark Mechanical.