MLV or Mass Loaded Vinyl isn't meant to get rid of vibrations. It's used to help offset resonating panels. When panels vibrate, they're acting the same as speakers and add a lot of terrible noise to your music. Remove 90% of resonation(to at least a lower octave) by adding 25% coverage of MLV(most people call it dynamat, second skin, raamat, etc.)
Next you need to get a closed cell foam to help isolate other items away from the resonating panels. CLD tiles are better for large flat panels(weight is your friend when treating rattles/vibrations). Otherwise, that wall idea works great!!
Ported helps too. Maybe prop open that trunk flap that helps regulate moisture and cabin pressure? License plate is key too.
