I use Dynamat its a name brand hi temp cheapest place is EbayI tried some form of insulation once, something like insulation used inside car panels, forget name of it now, and ended up cooking my coil. Where did you find this rubatex?? Will certainly give it a try. I also tried blocking up the area between dump box and seats which was probably not a good idea as it caused too much heat.
Did you ever get a chance to take a picture of how you installed your sound barrier? I am picking up a XTX next weekend and I need to do something to quite it down inside the cab. I am have the new AC heated installed and few other options. Can’t wait.I insulated the engine cover with rubatex insulation. It is a closed cell rubber insulation that you have to glue in. It made a world of difference in the amount of noise and heat.