Came up with this pretty simple idea, the design of the brackets took a little while to get just right. The key was to start with plain old fender washers...
Step 1: Washers!
They are inexpensive, easy to source, and have the perfect look and function when the sides are bent into position. The smooth curve of the edges will not snag on anything. The mounting points also work well with the direction of applied force, had I used angle brackets the downward force of any load would eventually deform the skin of the camper.
Before you commit to a particular washer (and buy a dozen of them) test bending one with a hammer. The first washers I bought wouldn't bend all the way to 90 degrees. They broke. The metal must have had some case-hardening, making them stronger, but brittle.
The brittle washers came from my local hardware store. The washers that bent up nice were from a dedicated fastener store, go figure.
I was tempted to make my own blanks with a hole saw, but I didn't have the right size saw. For a 1/4" rod with a 1/4" gap, the 2-inch washers worked great. If you needed something different and/or non-standard, cutting blanks out of flat stock with a hole-saw would be a good fallback. Plus, cutting them yourself would let you use exactly the material you want. For instance, a thinner gauge of aluminum instead of steel would have bent up better, and been plenty strong. The finished brackets made from washers are very sturdy, maybe a little overkill.
Whether you buy or make your bracket stock, get one more than you think you'll need. I started with 4 brackets for an 8' section, one bracket every 2', right? You also need one at the beginning, at position "0", in addition to 2', 4', 6' and 8'. That makes 5 brackets for an 8ft. rod. This type of "fencepost" error always gets me.