I don't know what it is called but Packard used a lot of tar like impregnated strip materials made of braided cotton welting and some flat cork/rubber compositions as well as ordinary sheet rubber strips. Not sure if it is the same stuff they used between the fenders and wheelwells in the trunk and carried that below the car. Packard only mentions something to the effect that asphalt based scientifically developed noise and heat insulation is placed in strategic spots throughout the car.
The stuff usually seen is a tar like substance which is blended in materials which can be paper, felt or jute based and comes in thin cardboard like sheets, thicker pads and probably rope form too. Plain jute is also used behind cardboard facings and carpet.
To replace the item in the trunk, some have used a cotton braided wick material such as
this one. I don't find anything larger than 3/4" but it does come in many thinner sizes. If something larger is needed perhaps a cotton or jute rope would work.
In the trunk, the rope like material should be out of sight hidden by the side cardboard or whatever finish material you choose so color might not matter. If you use a cotton rope material, perhaps a good coating of an upholstery type waterproofing liquid would be in order before installation. If under the car possibly dipped in or sprayed with undercoat material.
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