I use a short steam box for bending ribs for kayaks. The ribs are 1/4 inch thick and 3/4 inches wide and usually less than two foot long. I use a long steam box for steaming coamings that go around the cockpit of the kayak. The wood for the coamings is about 7 feet long and 3/4 inches by 1-1/2 inches in cross section.
Step 1: Materials list
You will need a coat hanger or a few feet of heavy wire as supports inside the steam box to keep the wood off the bottom so heat can get to it from all sides.
You will need a cooking pot. A two quart size is good for starters. You can fill it two thirds full and generate steam for several hours.
You will need a heat source. If you plan on working outdoors, a camping stove works fine. If you work indoors, you can use a hotplate. The hotplate I use runs 750 watts. That is hot enough for the three foot steambox I use and just hot enough for the 8 foot steam box.
Since first writing this Instructable, I have gotten my hands on a wallpaper steamer. It works much better than boiling water on a hotplate, mostly because it runs at a higher wattage and puts out more heat. If you can get a used one, go for it.
Step 2: Construct the steam box
Nail or screw the 4 sides together.
Close off one end of the box. Leave the other end open.
Screw a piece of plywood that is 4 inches wider than the diameter of your cooking pot to the capped off end of the steam box.
Cut a hole in the center of the piece of plywood so steam can get from the cooking pot into the steam box.
Step 3: Add wires to support your wood
Finding the hole at the far side by feel is tricky. Shine a light in the open end of the box and look through the hole that you are aiming the wire for. When the light disappears, you have found the hole.
Step 4: Add legs to support the open end of your steambox
If you're not that ambitious, you can just pile stuff under the open end of the steambox to raise it to the right elevation.
Step 5: Steam away
Keep in mind that some woods bend better than others. White oak, red oak, ash and poplar all bend well. Straight grain is important as well. If you have rain running out of the face of the board, your wood will most likely break where the grain runs out.
It's really easy to leave on the heat when you're done bending wood. Eventually, the water all evaporates, the pot becomes hot and the plywood starts smoking. Good way to start a fire.