you want use mm NOT inches.
also have a look at Openscad - things like that are doable in minutes and it only outputs fully manifold printable models.

I also don't see any holes in the case for you to plug stuff into the board. is that deliberate ?

How big is it ? if convert directly from mm to inches its 11.7 inches by 6.26 by 2.14
That's pretty big for a pi case.