I use dynamat super-no odor, cool foil surface over polyurethane.
I got it locally from pepboys auto. Any decent auto sound department should have it or similar. The stuff is heavy, so shipping can kill you. It is easy to apply, just cut to shape with scissors, and pull out the backing as you stick it down. Be sure that all surfaces are clean and oil-free. Don't expose more adhesive than you have to at any time, it sticks aggressively and doesn't come off easily. I used a small wallpaper seam roller and the flat rounded point of a table knife for the remote corners.
Don't expect any miracles. This stuff is for vibration dampening. It just stops the case from singing in harmony with the fans, and has virtually no sound absorbing properties at all. Won't do a thing to silence the scream coming from that high output 60mm black label Delta.....