I use peal and seal self stick roofing from Lowes. a lot cheaper than sound deadener.
Water proof and flexible. This is the stuff used under roofing.
Most of the stuff is black but I had some that was foil backed that I put in my Beetle and it made a huge difference.
I just cleaned floor and stuck it down. Then used heat gun to work it into grooves and dips. Put underlayment over that and carpet on top. What a huge difference it made.
https://www.lowes.com/pd/Grace-Cons...0-sq-ft-Rubber-Roof-Underlayment/3064819
