For best results download the latest OpenSCAD development snapshot of the desktop client from openScad dev snapshot Once installed, open the client and select edit>preferences : then open the features tab. check / activate the following features : fast-csg, fast-csg-trust-corefinement, fast-csg-remesh, Lazy-union

Once your file is open, press F6 to render the 3d model. then File>export and choose your prefered model format.