Find galvanized metal sheets in the desired shape and size. The planters should be at least 2 feet deep to give the plants' roots enough room to grow. Have the scrapyard cut the metal to fit for you. For example, have two pieces of metal cut 6 feet long and 2 feet deep for the length of the vegetable bed. For the width, use sheets that are 3 feet long and 2 feet deep.
Drill holes in the metal for the brackets with a titanium oxide bit. Hold the brackets up to the metal and use a marker to note where the holes need to be drilled. Each bracket has two holes on either end. They form the corner so you'll need to drill two holes on the top, bottom, left and right of each piece of metal.
Assemble the unit by screwing two brackets onto one piece of metal. Hold the metal upright and screw the brackets through the other corner piece. Use nuts to hold the screws in place. Repeat for all four corners.
Fill the vegetable box with the desired potting soil. To increase drainage, start with rocks in the bottom and add soil, compost and any other amendments on top.