Exactly and with the towel technique, since they are easy to move around you can quickly tune to the car, where the towels end up is where you probably need absorption. If you have cloth/ felt/ suede at your house you can use them too since each will have a different sonic affect. With my current ride I used rubber cement to attach felt to parts of the interior I didn't want a refection (the cement comes up easily so it wont leave a permanent residue). I color coordinated the felt so it looks good with the interior.

For the final product you can go to an automotive upholsterer give them the materials you selected for them to use and they will fab up a good looking mat with the best sonic properties for your specific build. They can stitch in your name, the cars model, your sponsors name... and give it a nice professional look.

This is one area where TA, DSP and EQ will let you down, but with my SHO I tried to do all the EQ with materials and fabrics... I think the EQ and TA is a lot easier and more successful.

edit: A LOT more successful at killing problem frquencies

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I don't know what your problem is, but I'll bet it's hard to pronounce.