Guy, we have ~10mm neoprene for our arch liners.
For those that want a spray on coating, wurth make a really good water-based spray on body deadener that you apply with a Schutz gun. You can get the guns cheap on Ebay:
Stone Guard GUN Fish OIL Schutz Under Coating GUN Body Deadner GUN PS 8 | eBay
The gun screws straight onto the body deadener bottle.
Alternatively, you can have a professional spray the Rhino Liner deadener which is tough as nails.