You will need to get some thickness to the material for it work. Line-X charges approximately $150 for this to be accomplished as they need to pep and take the wheels off. However, you will want a material that remains flexible so that if it ever does dislodge in a large piece it does not wreak havoc inside your fender well; products like Rhino have been reported to come of in large sheets.
Dupli-Color Truck Bed Coating is another thought for those who want to use a truck bed lining material, but I would apply it via the spray gun and air compressor (watch their video).
Second Skin Spectrum can also be used as an undercoating. I have not got around to coating our Z yet, but it is will done.
The issue is that the inside of the rear fender wells are like a drum; you need simply take your fingers and you can literally tap out a tune on the wells. There is a thin factory undercoating, but it is hard in nature, and adds to the noise. The ultimate will probably be a sandwich solution which will mean removing the rear interior pieces; no so sure I am willing to go to that extent. In a way, I am surprised that Nissan did not use their acoustic engineers to alleviate this issue, or to better address it; 2010 model?