Do you really need a steam box? Boiling parts in water serves the same purpose as steaming and requires a simpler set-up with fewer heat-loss issues. ‘Chairmaking in High Wycombe’ references boiling of chair bows and I have bent table legs, shaker boxes and hay forks in pots or pans of boiling water. Boiling require less equipment and has fewer pitfalls than steaming (boiling water is always 212 degreees), so if your parts are small and few, boiling is probably your best bet. Only the section of your part that is being bent needs to be heated.
Heat is the primary factor effecting how well your steam box works. The box should hold a temperature of roughly 210 degree; visible steam can be much lower in temperature, so make sure to measure the box’s temperature with a meat thermometer. Cold outdoor temperatures can make it much harder to get your box up to 210. I have bent green oak with steam box temperatures as low as 180, but this is a terrible idea and leads to much more breakage and requires more effort to bend.
Boxes have 4 main parts: the insulated box itself, the racking inside, the doors and a thermometer:
Wood: Curtis Buchanan’s box is made of exterior-grade plywood (CDX) and has a 4“ box inside a 6“ box with foam-board insulation between the two. The outside 6“ box merely protects the insulation from damage and can be made from anything you have on hand: sheet metal, solid wood, thin plywood, etc. My box is similar, but I used scrap foam-board with spray foam to fill in the gaps between the foam-boards. Avoid making the box from solid wood, which is likely to warp because the inside repeatedly gets soaked in wet steam while the outside remains drier.
Pipe: Round boxes have less usable internal space than a similarly sized square box. Sheet metal pipe is cheap, but holds no heat; make sure to insulate it well. Schedule 40 PVC will gradually sag with repeated heating unless very well supported (see ‘combination’ boxes below). Schedule 80 gaspipe or sewer pipe will last quite a while, but is expensive unless you can bum a scrap piece from a roadside pipe crew.
Combination: Roy Underhill uses a steambox made of 4“ pipe in a 6“ square wooden box, with spray foam insulation between the pipe and wood. This allows you to use any kind of pipe you like (PVC, sheet metal, etc) without worrying about heat loss or sagging.