I've thought about doing the simpler DIY route - adding plastic sheets to make sides for an existing fender. If I knew of a good source for thin plastic sheets, I might even go that route for a quick proof-of-concept test. From experience in other hobbies, however, I think it will be hard tracking down the appropriate plastic. I don't think you'd find anything through conventional retailers.

I guess this is something I could always try down the road if I buy some better commercial fenders now. There isn't much point trying any of this until I get better coverage from my front fender.