Shouldn't be too difficult for an inner fender. Doesn't have to look pretty but water proofing is a must. The hard part is finding the right material. Has to stand up to what ever the tires may pick up and throw at it. if you can find the right material shouldn't be a very difficult weekend project.
The diamond plate idea is worth considering. Plastic is easier but you will need to find the correct material. Certain plastics will become very brittle in cold weather and one good size stone will shatter or crack it.
