There are at least 40 different ways of mounting them. Some use bracing, some not. Some straight , some curved. Mine are curved and need no support. Some painted some not. Here is what I have. They are made of a fairly rigid aluminum. They were polished until they met the asphalt leaving a friends driveway. Straightened them and took a Scotchbrite to them. I added a strip on the inside to bolt to. Probably could have used fender washers as the fiberglass is pretty thick there. My pattern was furnished by another forum member.
Here ar a few of the different ones.
I don't have many good pics of the spoiler, and it will look different in the spring as I am remaking it. Standard graph paper was used for the measurements.
It is mounted with large button head screws and fender washers, with nylocs on the inside with fender washers. There are double rubber washers(as big as the fender washers) between the spoiler and the body to save the paint. The vents are based on where the fiberglass upright supports are for the wheel wells on the inside(that is what they are venting). The last bolts outside of the vents are smaller than the ones on the inside due to closeness to the body edge. The length of the spoiler varies with each body. It can be as tall as you want but most only go slightly above the body spoiler. Some just put two holes at the mounting point and cover the undrilled portion with the fender washer to make it look adjustable.
Hope this gives you some insight.