I was going to say, similar to the old rubber/asphalt mud flaps, I think. I've replaced mine with better ones off donors in the past, but that supply has dried up so for now, I have to make do with the one that's in the car.
I had a couple of rips on the skirt in my 'Anny', so I took it out, scrubbed it as clean as I could, let it dry out, did my best to encourage some flatness, and then used asphalt roof repair compound in a tube, then laid on pieces of plastic window screen to knit/strengthen the rips, smoothing as I went, sort of like laying out fiberglas & resin. I didn't try to do all the work in one step, instead doing the work in stages so I could judge thickness, let it dry a bit as I went, etc.
The roofing compound is serious stuf, messy, and you need to have sacrificial spreaders and several pairs of rubber gloves handy as you go along. I had to clamp the rips to a work top until the mesh firmed up it's grip, using blue painters tape on the clamp surfaces. You need to be really patient with this type of repair.
Let it dry in the sun for a few days until it lost it's tack and firmed up, touched up as needed, then re-installed and so far, it's holding shape. I was hoping that the repairs wouldn't be too noticeable, and while they blended well, when the roofing mastic I used dried, the result was a contrasting very shiny high gloss black, not flat, oh well.
If you wanted to DIY a new one, using the old one as a template, you could look for a thin (?) rubber door mat, or, proper thickness conveyor belt material, neoprene sheet, etc.
https://www.amazon.com/Conveyor-Belt-Material/s?k=Conveyor+Belt+Material