Hey sorry I've not been on here in a while. I used white tiger seal, it's a rubber bonding agent for body kits and stuff. The car I got it off it was held on by a few clips and bolts but mine from the factory without the spoiler didn't have those.
If you do use tiger seal, I'd reccommend getting black instead, and putting it quite far inside the underneath as it spreads and is visible on my car. Put it on, hold it for a couple of minutes and get someone to tac painter's tape to hold it on overnight.