If you happen to have some sheet metal, a 2d profiler (waterjet, laser, sheers), some clamps, and a welder it's possible to throw together a planter box that hangs over a railing. All in all this is a reasonable, afternoon project.
However, I don't actually recommend using this planter: for one it gets rusty, and two I was informed high iron content is not healthy for males. As I understand it, some plants do uptake minerals from the soil, so you may end up with high iron tomatoes which may not be healthy. This instructable is probably mostly just helpful for suggesting an alternative bending technique to a finger brake.
Step 1: CAD Design
Draw up a 2D profile of the box(attached are solidworks files for editing and dxfs just for cutting). I made use of segmented bends to make the bendline eaiser to find and easier to form with a simple finger brake. I had the luxury of a waterjet to cut the profiles.
Step 2: Bend Flanges
Once the parts are cut, you can start bending the flanges. I started to make use of a finger brake, but the range of motion was fairly limited: as soon as the flanges begin to bend they sweep up into the way of the brake. So, I decided to make use of a rigid piece of metal and some clamps.
I clamped the sheetmetal between the worktable and a rigid piece of metal, then I took another thick, rigid piece of metal and used that to evenly bend the flange. I put my body weight on it as bending and it came out fairly decently.
Step 3: Fixture, and Weld
I was only able to partially bend all of the flanges into place with the manual bending technique. So I then fixtured the setup to a welding table and clamped it down as securely and evenly as possible. Then, while holding the pieces in location with clamps and my body weight I was then able to tack weld the corners together. After tack welding I was then able to run beads along the joints to secure them.