Step 1: Find Materials
The other main material you'll need is flat steel. I got mine from a metal shipping frame. You might be able to get these from construction sites or motorcycle/scooter shops. This material is sturdy enough to hold up under pressure, but not too hard to shape or cut with a hack saw. You might also be able to cut down some thicker sheet metal or fold thinner sheet metal into a usable thickness here as a substitute, just be careful of those sharp edges. My foot has a 4 inch scar from when I wasn't once. Wear shoes and gloves.
As for fasteners, you'll want to keep an eye out hose clamps, nuts, bolts, and zip ties. These don't have to be of matching sizes, though it's nice if they are. You can likely hit a dollar store for these if you need to, but that would be going against our $0 cost. Don't hang around in a dumpster with an angry owner around trying to free up a couple bolts though. Do whatever works best for you.
This simple design also lends itself to subsituting parts if you can't fine one or another. You could hammer some metal piping flat for your frame metal, use bamboo and bailing wire to make a rack deck if you can't find a metal grate, wrap the arms and U bracket together with hose clamps or tape and/or baling wire in lieu of being able to drill holes or find bolts to hold them together, and obviously you can replace zip ties and hose clamps with anything from baling wire to string to nearly whatever you have handy that'll hold it down steady. The beauty of doing things yourself is that you don't need to play off of someone else's materials list if you can substitute in something you have that'll work for your needs. I highly encourage it.
Step 2: Ready Your Materials
The flat stock will need to be cut loose from it's donor structure as well. I borrowed a portable band saw for this, trying to keep as much of the material uncut as possible. Using a grinder to grind welds loose would help to keep your full-length pieces of material. The stock I wound up with was 4 pieces of (presumably) mild steel, each was 1/4"x1"x15".
If your materials are overly rusty you can use a wire brush, sand paper, or electrolysis to clean them up.
Step 3: Shape Your Materials
The frame is 4 pieces: the main arms, which support most of the weight and are attached to the back axle on either side, the U brace, which ties the arms together, and the spine brace, which ties all of that to the frame. The measurements of these will change based on what your bike's tire size is and what you're rack's width will be.
Note: I suggest drilling all holes in these parts before cutting them, as doing so after is more difficult.
You can bend the pieces out on any hard surface, though I find a vice to be best since it holds the piece steady and lets you get nice right angles and corners rather than more rounded ones. A picnic table/workbench and a C clamp is a good alternative as well. Make sure to bend the main arms at the same time so you don't wind up with one that's more heavily angled than the other. Compare them back and forth frequently if you can't do them both at once since having to bend one back too far will fatigue and weaken the metal.
Step 4: Paint Parts
Step 5: Assemble!
1: Loosely attach main arms to U brace and slip onto outside of axle thread.
2: Loosely attach spine brace to U brace and frame.
3: Take off one main arm, then the axle nut on that side. put the main arm back on the axle, with an oversized nut under it if needed to get around chain guard bolt on right side. Loosely screw on axle nut.
4: Do same for other side.
5: Tighten frame bolts, making sure the whole frame is square as you go.
6: Re-tighten axle nuts, making sure the chain has good tension.
7: Set rack on frame, use zip ties and hose clamps to hold it in place.
8: Test ride bike, making sure everything is to your liking. This is where I figured out I needed to cut a bit off the front of the rack to keep it from hitting my legs as I stood up.
9:Make any adjustments you need to.
10: Enjoy and post a comment with a picture of your rack. You can never get too many pictures of nice racks. :P
You can check out my other projects at http://thestuffimade.blogspot.com/ if you like.