I'm using a different material inside the doors and between the package area seat back and the engine. It's 3/8" thick heavy rubber mat like they put under the workout machines in the gym. I got it from Amazon and it's called Rubber-Cal recycled floor mat. It's really heavy. I tried a piece out on the driver's floorboard but it was a little bit too thick for the way I've got my seat tracks set up.
Mike