I'd go for rubber fan mounts. Good quality ones, they'll absorb the vibrations and keep the rig pretty silent and vibration free..... The fans aren't going to fall onto anything if you fix them correctly and securely enough.
Something like these
Fixed like this
Available from http://www.gelidsolutions.com/products/index.php?lid=2&cid=11&id=43http://www.acoustiproducts.com/en/ultra_soft_fan_mounts.asphttp://www.nexustechnologyusa.com/c/ntusa/mountingkit.html