I find it offensive on principle that Toyota bolted fiberglass flares to these rigs. Don't get me wrong, I think they look great, but they're such a weird mish-mash of friction pins and bolts, it's just a disaster waiting to happen. The gaskets break down, the paint gets scuffed, plastic pins pop out or break, and then a couple of bolts damage the body panels when the flare finally gets caught on something.
It's a lot of work to de-flare properly. Welding, grinding, filling, sanding, and repainting. I'm pretty sure you could get away with using polyester resin and a couple layers of fiberglass cloth on the back side, then fill the top with reinforced Bondo. I doubt you'd ever see that fail in the real world.
I'm considering using some universal rubber flares like these:
My tires currently stick out at rest. These is technically illegal, but nobody over the age of 20 ever gets ticketed for it here. More importantly, as an Oregonian, I absolutely despise people who throw up huge plumes of that godawful winter road mist, is it's important that I find a way to have flares and some degree of mud flaps.
Long term, I might refinish my flares and make some slightly flexible copies that could be held on purely with adhesive. I doubt there's enough force exerted on them at road speed that modern adhesives couldn't hold them in place.