Stopping trunk lid rattle can be somewhat of a pain. Your best bet is going to be adding mass, which would by using a product such as Audio Technix Sound Deadener - Better than Dynamat - Home offers. Using great stuff in the gaps between the braces and trunk panel itself helps as well, but I haven't noticed a lot of room to do that in my car. In all honesty, it's going to rattle (edit: It's going to flex) on low notes, assuming you have a somewhat potent setup. Mine actually makes a strange squeaking sound on the lowest ones lol. Oh... tighten your spoiler bolts too. Get in the habit of checking those every other week or so. Helps a ton. It's fun when your whole back end moves as much as the trunk does