Using real hardware is the best way to go about securing it down although it's not as easy at first.A lot of people have installed them. Mine was done by the dealer before I even saw the car, but my recollection from various posts is that the spoiler comes with a template and two fixed bolts in the bottom. Some have filed off the bolts and used an adhesive (not Elmer's) with no reports of airborne expressway spoilers. You want to get it from a dealer parts department already painted.
Any recommendations for an online dealer website - I'm not in the US.