I'm not very knowledgeable about fabric, but the texture of the underarmour was close to neoprene or a soft vinyl.. Perhaps you could find something suitable in the department of vehicle upholstery? I'm not sure where you'd shop for that though.
The general purpose (black) santoprene on that site looks perfect. Being a thermoplastic, it could easily be heated and vacuformed, making it a good candidate for mkVI underarmor details. And it's only 10 times thicker than a sheet of paper.