Just yesterday, I installed some of the original carpet back onto the walls of my Casita
so have a miniscule amount of experience that I can relate.
All of the materials used almost universally by the fiberglass manufacturers is some kind of man made fiber over a foam backing. Rat fur used by Scamp
or carpet used by Casita
are my examples. The importance of the foam backing is that it provides an insulation value and a vapor barrier. The vapor barrier function is an important one in that the fiberglass bodies of our trailers will sweat a little bit. Without the foam layer, that moisture will soak down into the fabric and likely result in mold. The rat fur and carpet used come from the fiberglass boat industry. I have done searches there and came up with new materials available for purchase. From a visual standpoint, I love the look of ensolite but I hear that it isn't available anymore.
I used regular hardware store contact cement to glue the carpet to interior. I chose a warm, dry day, following the instructions on the can. Brush or roll the cement to both surfaces, let it dry for 15-20 minutes. press and roll in place. When pressed in place, the carpet stuck instantly and securely to the fiberglass.
Patterns I have seen no patterns in my searches on the net. Maybe someone else knows of one. I can say that with carpet, and probably rat fur, that the material is fairly forgiving along the seams. If you cut the material a little bit tight, the seam gets lost in the piles. If you screw up and get to wide of a joint, you can glue a strip in to fill the void. It will get lost in the sea of piles.