3M tape worked fine for me, no gaps or loose fitment. as long as you use the bolts on the bottom and use some type of adhesive on the top, you'll be fine. the lip is very light (I was expecting it to be heavier) so it doesnt take much to have it adhere to your bumper.
i'm not really sure that this piece was truly designed as something to reduce drag or to provide more stability. personally i think more cosmetic than anything.