duplicate the model, invert poly's, merge polygons together... doubles the poly's, but closes your mesh

First check why your model has so many open meshes. It is unusual for professional models to have open meshes.

If its meshes are really broken and can't be fixed, create LOD models. With the Pro Edition, MED does this automatically, with lower editions you have to create such a model manually. Make sure that the meshes of the LOD models are closed.