Step 1: Step 1: Making the Base
Like I said, I used and old skateboard rail I had (I'd recommend looking on craigslist and finding one for this project, it'll probably be cheaper than buying the metal on it's own, and it will make this project 10x easier.)
So I took the feet off of the rail and cut some half circles out of it so that the bed cleats could sit in there and the foot could rest on it (we'll bolt the feet to the cleat later). To start, put the cleats in the back of your bed rails, any spots you feel are best. Then set your rail foot on it, trace out where the half circle cut should be and then cut it. Once you have done that, set it back on the cleats and from the bottom, using a pencil, mark through the holes in the cleat onto the foot. Drill a hole as wide as whatever bolt you plan on using into that foot (there should be 4 holes in it, 2 per cleat, 2 cleats per foot). Now put the foot back on the cleat and put your bolts through, this will keep it locked on.
Step 2: Step 2: Making the Frame
Now that the legs are done, you'll have the actual rail portion left over, you'll notice that it fits perfectly onto the feet because that's what it was designed for.
Measure how high you want the actual rack to sit and cut two lengths of that measurement. You'll notice that I used the outsides and put the spot where the legs connected up top (I'll go into that in a second). Go ahead and drill a hole through each of the rails once you've sat them on the foot (the foot should already have multiple holes drilled into it so when it WAS a skateboard rail you could adjust the height of the rail). Bolt these vertical poles to the feet. All that's left is the middle section. I purposely left the spot where the feet connected originally at the top so that there is about 6-9 inches extra vertical so if something was to slide around it would slide between these two vertical stops and not slide off of the rack.
The next step is to get another piece of metal (if your rail wasn't long enough, which it likely won't be, I think all in all I used about 9 feet of rail and my rail was only 8 foot long, but lucky for me I had two, but if all you have to buy is one square piece the width of your bed, you're not too bad off.) So now you want to measure it out see how long of a piece you need between the vertical poles. You can adjust this how you want, I made mine so that the poles were 100% vertical so made a 90 degree between each vertical, so basically when I'm storing it the feet sit flat on the ground not cambered at all. So go ahead and measure that and cut another piece to that length. Weld that piece in between the poles (using what WAS the foot mounts). NOTE: You could also get a piece that's slightly larger than the feet and can slide over them and bolt those on, but I wanted to weld it.
After that you're done (other than paint). AS you can tell it's pretty strong, it distributes the weight it carries between the 4 bed cleats. I weigh about 170lbs and I didn't notice any sagging whatsoever of the rail system.
Step 3: Step 3: Paint It and You're Done!
Through some paint on your new rack and test it out! The last photo is one of my truck with 2 kayaks on that rack and my camper.