Face it, the acoustics and speaker enclosures inside a Jeep are bad enough that using polyfill, boom mats, etc. won't make a real improvement in sound. At least no more than what the placebo effect will cause. Especially inside a soundbar or behind the front speakers. I wouldn't get caught up in it, it's like worrying about selecting the 'just right' expensive Persian rug when you live in a $9/night hotel room over the bus station.