So I modified your idea to make a bent PVC fender. It installs easily and holds in place with a 1/4" x 4 1/2" Stainless Steel hex bolt. There is no need to put a nut on the bottom under the fender rail unless you want to leave it on when you are running.
Used 1/2" Schedule 40 PVC. It moves side to side only a little bit and that could likely be addressed by changing the length/shape of the pipe that runs underneath the rail. If you wanted easier install, you could make it without the hook at the top that goes into the foot well.
Here are some pictures of the rear fender, I still need to make the front one. They feel very sturdy.
Guess there's not much point in painting it