Yeah, it has to be hollow. It needs to be made of pyramids/triangles because if you try to acheive this with flats you'll get invalid solids. In all, it's 128 solids and 64 rendered faces.
Oh, and its 8-sided...I can't count
Theoretically, I could make it actually hollow and player crawlable, but it would double in brushes and faces.
EDIT: There, I actually did it. It's hollow inside now, with double the amount of brushes and faces.