Looks like a trip to Joanne Fabric or whatever is there, is in your future, to sew up a new one. Looks like maybe 2 to 2 1/2 yards of ripstop ought to do the trick. Make a tube for the coil spring and baste it in place before sewing it in place. It's a thought at least.

I'd offer my sewing machine but you could sew it by hand by the time I could travel that far, sorry.

It's the ultraviolet component of the sunlight that has done it in. Thermoplastics don't like UV very much. And once the added stabilizers have been used up, damage progresses much more rapidly.

Generally, the thinner and more flexible a thermoplastic is, the shorter the expected lifespan in outside conditions.

Thermosetting resins can usually withstand it much better because they are cross-linked.

Vester --- Great idea! I actually tend to have big hunks of fabric around, and I could see replacing the plastic with fabric.

Roland --- That's a lot of good information. I'll have to remember that thin, flexible plastic needs more protection.

I'd suggest a canvas as a suitable material. But if you don't like cotton, you could go with hemp, if you can get it. (I'm not sure linen is suitable, it's fiber can break when creased and it absorbs a lot of water)

And maybe paint it with linseed oil to make it water repellent? Be careful though, the drying reaction of linseed oil is exothermic, and can cause a fire if you leave rags soaked in linseed oil in a pile.

You can even order it online. Don't bother with the more expensive s-glass yarns. Stick to the cheaper e-glass. A satin or twill weave will be more pliable than a plain weave.

If you impregnate glass fiber cloth with epoxy or polyester resin, it makes very strong and light products. If you stiched a bag out of glass fiber and painted the edge of the bag with polyester and kept it open while the resin cured, that edge it would keep its shape after the polyester hardened.

You can do interesting things with fibers and resins. See e.g. the knotted chair.