I live in a rather old house, and in the bathroom the heating register is oddly busted up. I'm wondering if it would be a safety violation to fabricate the outer frame of the register out of wood, that would match the vanity furniture in the room. I would retrofit it to the metal grate which holds the blocking mechanism.
I second what Ed said with this addition, air flow is paramount. You want to do all you can do maintain or enhance the air flow through the register, i.e. taper the back your fins so the air stream does not have to hit the blunt surface of the bottom side of the fins. And keep your gaps as wide as you can without compromising the look or safety.