I found plastic interior wall panels on Craigslist for $125 dollars (SCORE!) and yesterday I installed the panels on rear doors and and passenger rear walls after I filled the wall cavities with Roxul insulation. In addition to keeping us cool I think it will help reduce the noise level. I installed a sound app on my phone so I’ll test the noise level when all the insulation and wall panels are installed but so far there seems to be about a 1.5 to 2 decibel decrease.