I say just velcor or somethign and experiment with the different location. You might find a different one that sounds even better. Im sure different sets of speakers will behave differently in the given locations.

That being said with my alpine set I have had my tweeters in different locations and ended up with them up on the dash for a few reasons. 1. So it wouldn't be as effected with stuff/ people in the passenger seat and 2. to bring the soundstage up. The tweeters were kinda bright so i had to compensate. Imaging did suffer a bit but not too bad. Every locations seem to have their own trade offs. I would just try different places and decide where you like them the best. Aiming of the tweeter will also have a significant impact on the sound. That has jsut been my experience.