What should I use under the vinyl to smooth out the interior?

I am referring to the armrest and well cover pieces (I have a convertible). It looks like the stock material was some sort of matting cloth. I hit my local fabric store to see if they had anything similar but all I found was some matting material that they use inside of quilts/blankets. Is this the same stuff?
It looked a bit thicker that the stock matting.

ive seen foam rubber used alot in different thicknesses and densities