I have been collecting trampoline fabric(polypropylene) for a number of years. The stuff is uv resistant, wicks away water/debries, and can be found very cheap used. I have this fun idea of stretching it over a ultralight space frame here eventually.
But the idea of "shaping" the fabric it by heating/shrinking into a relative shape by sandwiching it between a upper and lower mold then heating those molds has also crossed my mind.
I am experimenting stretching the fabric over fenders and hood of a square body in April. Many things are put on hold until I am in a larger place with less "restrictions" on my hot rodding activities.
Fiberglass is fun. But most people don't realize just how heavy reliable fiberglass panels end up being.
If you could "shape" trampoline fabric (polypropylene) into a rigid structure you have potential for not only a lightweight highly uv resistant panel you could do it cheap and quickly. Less cure time, sanding, and no need for paint.
As far as mashing/melting. There is potential for casting or meshing into a shape. But I would be concerned with how the chemicals would react and the potential for cracks in a large structure such as a fender. One of those things where "printing" items(or using a compatible fabric) may yield a mesh structure for you to lay your mash upon.
That being said. Keep experimenting. If the stuff is cheap and your enjoying it you will have fun in learning. Just take proper precautions as burns do happen and your dealing with some stuff you should not be directly breathing in.