Grommets in the bolt holes?
Here is what I did, others can do what they want. But this works for me, and many others.
Test fit the spoiler before paint, you may need to modify the trunk lid to make the new spoiler holes line up...but your really making new holes where the spoiler holes are on the new spoiler. (poor quality control) Anyway...
when everything is cool, and fits good. I use spacers under my spoilers to lift the spoiler up off the trunk lid about 1/8" or so, that way water and large items from trees, or bugs can be washed out from under it easy. Use what you want, many have even used the garden hose replacement o-rings you can get at any store that sells stuff like that. I the seal both sides of the spacer over the hold to the trunk lid, set down the trunk lid, and secure, and let dry in a safe place for 24 hours and not letting it get wet.
What to use to seal?
I use CLEAR silicone, that you can find in any plumbing section. "Plumbers goop" works pretty good too.
~F~