I have two loose spoilers actually. A dealer one and an aftermarket. Mounting would be easy, just drill holes and screw it together. The hard parts are measuring where to drill holes and how to wire the brake light.
When I eventually get around to painting and mounting my dealer option spoiler, my plan was to measure the width of the trunk lid; then cross reference that with the width between the spoiler's mounting points. At that point you can use simple math to determine the distance from the edge of the trunk lid you need to drill holes. I would also use some kind of paint on the holes after drilling them to prevent rust.
Your other option is to go back to the junkyard and measure the trunk lid of the car you pulled it from since it already has holes drilled.
For wiring, your options would be to either splice into the brake light wiring for the main tail lights because those wires should be accessible in the trunk area; or, run a wire up to the 3rd brake light where you could either use a pigtail in the bulb connector (since the spoiler light would legally be able to replace the 3rd brake light) or splice into the wiring for the 3rd brake light and retain the bulb.