Here in sweden you often see bathroom lamps with a plastic dome that screws in to a big porcelain socket. They would be ideal.
This is what i mean, kinda, you often see them mounted on walls, and with white sockets. Must be made in china since they're so common:
I ebay'd lamp shade sphere and got this: http://www.ebay.com/itm/10-REPLACEME...item61cb87ce16
Should be able to fit a metz into that. 10 inches, that's a big diffuser! And 14.95 for a 10", 4" opening, white, and made in the USA on top of that.