Jeri asked:
"Now if I can invent a "cradle" for my fiddle with a big magnet on the back so I can slap it on the air duct, I'd be happy."

Is this air duct warm? cold? the temp change might not be good for your instrument. If temp is not a problem, then you COULD use a magnet from:
http://www.leevalley.com/hardware/page.asp?page=42363&category=3&SID=&ccurrency=1

The problem with magnets is their rated strength is reached only by pulling at a right angle, ie, perpendicular to the attached duct. At any other angle the force is much less.

An alternative for you might be construction grade adhesive. This is sold at hardware stores, lumber yards etc. It comes in a tube suitable for use in a caulking gun- also available at the same places, for a couple of bucks.
This adhesive is used to attach things to concrete walls: it's got holding power! a small dab of it on the back of an instrument wall hook-if it's flat- should work fine, provided the duct is not too hot--but you wouldn't want to hang a fiddle against that anyway.