Not to be a complete idiot, but why do you want to use a softbox/diffuser when shooting closeup. If the flash isn't too far from the subject it is effectively a large, not a point, source of light and will give fairly soft lighting. If I weren't non-digital I'd show you some scans, but since I'm not you'll have to take my word for it.

If your problem is specular reflections, a light tent -- a diffuser around the subject, not around the flash -- is the classical solution.