Some trailers have the top of the wheel well area just covered with Darco over the plywood. A spinning, parted radial belt is going to chew through that faster than you can say "Oh ship, my TPMS says we have a flat". A Plastic liner will last a few turns longer, maybe enough to get pulled over when you see the smoke and debris in the mirror. A metal covering will last for miles, probably until the belts have left the tire and all that remains are the sidewalls or the bead.
I used 16 ga 404 steel. That was maybe a bit too thick but I have the tools to cut it and drill. I glued and fastened it into the wooden frame and then, after caulking the joints, sprayed box liner on all the steel. These modern, high durability steel products can really offer excellent durability in thinner material. IIRC, when I helped my buddy reline a garbage truck compactor, we used 1/8" plate. My welding table has the same, .125 404 and it is like I installed it yesterday. Just thought I'd throw this out there for others that may wish to DIY a wheel well upgrade. It gives a lot of peace of mind.
