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.
Step 2: Get Square "mandrel" Stock, Start Bending
I lucked out and had a chunk of 5/8" aluminum, but I could have used easier-to-source 1/2" steel too, just left a 1/8" gap when doing the 2nd bend.
Easy-peasy, mark your lines, clamp securely, hammer down (slowly, gently and evenly) to 90 degrees. Flip it over and do it again.
A sheet metal bending brake could be used with lighter material, like 20-gauge aluminum, but the weight of the steel fender washers (12- or 16-gauge?) would be too much for any brake I've ever used. A press brake could work, but I think the blanks would be hard to hold safely. I guess if you have a press-brake, you probably wouldn't be on Instructables though...
The vise-and-hammer approach works fine unless you are going to be making hundreds of brackets.
Step 3: Drill Holes
Next drill the holes for your fasteners, screws or rivets. Drill a tiny hole through the rod for a cotter pin to keep the rod from slipping out of the brackets. I only used 1 cotter pin, that's plenty.
Clean up each drilled hole with a deburring tool when you're done and ALWAYS center punch your position before you drill.
Step 4: Prepare Metal Rod, Paint and Install
I also made a 12' length for the opposite side, up above the windows. That will be nice for attaching a tarp for shade, an outside shower curtain, or holding a portable water jug up high for filling the water tank.
I'm lucky, I have a small steel supply and welding shop near my house, they don't require minimums and have long stock. For this project I bought a 20' stick, cut it in 8' and 12' sections with a bolt cutter. It was less than $3, sold by weight. Hardware stores have those bins with various sizes of round rod, square tubing, angle, etc., but usually only in 4' lengths. You could piece it together with shorter segments, but the long, continuous run is my preference.
File or grind the ends of the rod square, chamfer the ends.
The washer-brackets are silver, the steel-rod a dirty-gray, so I gave everything a shot of white paint to match the camper. Don't put the paint on too heavy or the brackets won't slip over the rod.
Attach the ends first, then measure evenly for the in-between brackets. I started with sheet-rock screws to hold it all in position, make sure it looked right, then drill and install pop rivets.
The only thing holding the rod and brackets to the camper is the skin of the camper itself. This is a thin material, time will tell if it is strong enough to deal with the brackets and what I hang from the rod. Having the load spread across 10 rivets (2 in each of the 5 brackets) should be good. As long as I don't try to hang a hammock or rappel from it I should be fine.