I tried some of that American spray-on stuff in an aerosol from Demon Tweeks. It sort of worked, and it was easier to apply inside the doors but it will never do as good a job as the panels shown above that absorb the noise energy properly and dampen the panel vibrations. I figured it might also be a useful anti-rust spray if nothing else.
Because its American its a 1/4 thick they also do 1/8 and 1/2 inch. But the dynaliner does come in big roles and made all the difference with the doors