I have used welder's ground magnets from Harbor Freight that will hold
almost anything. You can see one mounted to the outside of my van top
support a small kitchen-side awning at

http://www.knology.net/~felder/Vanagons/

They hold a 12v fan on the dash, and also you can stick the fan to some
vertical surface in the van and it holds there too. Don't try this
without some headliner material or similar glued to the bottom.

Jim