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.