Small design bug:
Thus the Pi fit’s exactly in the bottom part, there is no space left for the air to exit. You can test it with a cigarette ;-) So at least there should be a space to the side walls in a way that the airflow passes the top of the pi and can exit between housing and board.
in that case - I would recommend some spacers also, before mounting the case to a surface.
in other way - nice job. The fan does fit inside also.