I'm not sure if this is the answer you need. From my experience the best stuff to use is a made for purpose product. I could not find it at a retail shop because there is not much retail demand. It is usually used by car restorers and upholstery shops. It is the same stuff that is used with "sound packages" from the factory. it looks like its made from shreded rags and scrap threads which seem to be mostly man made fibres. It is not the same stuff they use in homes. Try auto upholstery shops. I found mine at the upholsterers supplier. It shouldn't cost more that $30 for the amt you need for an MJ cab. Best results if you glue it to the carpet, NOT the floor, or at least not all of the floor.
Mask off one inch around the edge on the back side of your freshly shampood and dried carpet and spray it with contact cement. Also spray the underlay with contact cement. Leaving the edges without backing allows you to tuck under sill plates, under the dash etc. Same applies for around consoles... don't applying the backing right to the edge. The end result in in appearance, sound deadening, and comfort are worth the few bucks it costs. Jute can get real smelly when it gets wet.