Nice work 25th! Even though as you state the deadening material that's designed for this sort of thing works better, you are still making progress. And from your comments, you can already tell a difference. The idea of using it on the plastic panels as well will certainly help, good decision.
Certainly not meaning to be a know it all, I've just been doing this sort of thing a long time. When you try to build competition audio systems, its a must! Heck, back in the day (20+ years ago), when we didn't know any better, my dad and I just used foam mattress topper material we found at the local dept. store along with the jute type carpet padding you are using. It turned out very well, especially when you factor in how little it cost!
Great job, would love to hear your thoughts when you are all finished. Best of luck!