A steam box is a handy way to heat wood so that it becomes more pliable than it is at room temperature. Once pliable, you can bend the wood into shapes that you couldn't bend it into cold. Size of the steam box depends on the length and diameter of the wood that you intend to bend. In general, it's good to make the steam box just a little larger than the wood that you're trying to heat. The smaller the steam box, the faster it heats up and the sooner you're bending wood.
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 5/8 inch or 3/4 inch plywood in the appropriate length. Thinner plywood would work but loses heat too quickly unless you add insulation to the outside of the steam box.
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

Cut the plywood to the dimensions that you need. The interior of the steam box should be large enough so that when you load it up with wood there will still be an inch of space around the wood so that the steam can get to the wood.
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

Drill some holes in the side of the box to run your wire supports through. Keep the holes the same diameter as the wire to minimize the amount of steam that escapes or the need to do a lot of caulking. The wires will elevate the wood in the box so the steam can get to all sides of it. Cut wires 4 inches longer than the diameter of the box. Run the wire through the holes and bend over the ends so the wires don't fall out.
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

Set your pot up on the heater that you will be using. Screw some legs to the sides of the open end of the box so that it is level with the top of the pot. The level steam box is important so that the lid of the steam box makes a good seal with the top of the cooking pot.
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

When you're all set, fill your pot two thirds full of water, turn on the heater and wait for the water to boil. After the water has come to a boil, the steam box still takes a little while to heat up. Once you get a good flow of steam coming out of the end of the box, you're ready to heat and bend. One quarter inch thick ribs heat up in under 5 minutes. Thicker stock takes longer. Heat has to travel from the surface of the wood to the interior. Rule of thumb is 15 minutes of heating per 1/2 inch of thickness.
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.

Caution-Danger!
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.