I'm cheap !!
First I used spray on "boom mat" noise insulating undercoating... 2 cans for $26... bought on ebay... more than enough to do the roof, and floor. Next I bought and layed in "conventional" carpet pad that I bought at home depot... I think is was like $28 for a roll large enough to cover the entire floor. Cut it to fit...made the floor nice and thick!!
Seems to work well...As to sound...hard to tell when using Flowmaster 40's for my mufflers, and as far as heat is concerned... seems like the combo of the two keep the floor from getting hot.