Like Don Stith, I capitalized on the work of John Bivens, and borrowed heavily from his article in "Rifle" magazine on the subject of damp boxes. I built mine out of some 3/4' plywood shelving I recycled from the local dump, so I made mine 15" square and 66" tall. I built an asbestos lined box in the bottom to house a 300 watt incandescent light bulb in a ceramic fixture that heats a stainless steel bowl immediately above it, in which I add 2" of tap water. I found that this system, even with the dimmer switch which controls heat, produced too much humidity, so I half covered the bowl with a piece of plywood. On the inside about half way up the box I attached a 60 watt bulb in a ceramic fixture, and these two bulbs are on the same circuit as the dimmer switch. When I found the optimum heat and humidity, I marked the dimmer switch for future reference. My basement is dry - in the winter 7 % Relative Humidity, so the box is mandatory. The inside dimension permits me to brown barrels up to 52" long. In the ceiling of the box I have attached cup hooks so I can hang the barrels, and thereby they do not touch the side of the box. This also permits me to rotate the barrels 180 degrees after each application of browning solution, ie: every three hours. Barrels usually require 5 or 6 applications to get the colour and depth I want. Often, I cannot get the job done in one day, so I remove the parts from the box and leave them on the bench overnight. This always evens out the rust and yields a perfect brown.
This box is heavy, and like many of you, my space is limited. I have a wall behind the box that I use for a tillering wall for my wooden bows, so I mounted wheels on the bottom of my box to make moving it away from the wall easier. This box has served me for about thirty years.
I built mine vertical, 'cause John did, and it makes sense, because heat rises. to further control the amount of moisture in the box, I bored a couple of 3/4" holes on the side of the box to allow moisture to escape. If the humidity reaches dew point, drops of water will form on your parts and ruin the job.