I'm thinking about something like this:
B_and_D - I also don't like the idea that dirt, rain, and drips from the waste tank would land on this cage. What do you mean, "it would have been better if I had folded the sheet metal over so that it was folded over facing the bottom instead of the top."
This design could probably be simplified a bit to minimize cutting, but I'm thinking of making this out of something like 14 gauge galvanized steel, and all joints welded. (I've actually been thinking of taking a welding class, but it's hard to find a class that is just designed to teach you how to prevent hurting yourself and doing a decent job vs. becoming a welder by trade.) I'm guessing the cost will be around $100, including welding labor and the added frame member. Definitely more of a premium vs. your $15.
I'm also concerned that this design may also "catch air" as I'm driving down the road.
Both of these problems would be ameliorated with a true cage-like design, perhaps made from 1" tube steel. It would be stronger than sheet metal, would allow water and dirt to flow through it, and wouldn't catch air. But there are alot more weld joints, so I'm guessing cost would be much higher. I thought about trying to modify one of those hitch-carriers to function in this manner, like this:
...but none that I could find were deep enough to span the width of the tire and thus provide adequate protection.
Sigh...sometimes I feel we are doing jobs the factory should be doing....
Dave