Richard: I built my box according to John Bivens' instructions, ie: 15 1/4" x 15 1/4" x 66 1/2" (found some 3/4" ply shelving at the dump one day.. I have a 150 watt bulb in the bottom to heat a stainless steel bowl of water, and a 100 watt bulb on the side about the middle, both on a dimmer switch to regulate the heat and humidity. I drilled three 3/4" holes on one side at the top to vent. You don't want to hit the due point where droplets of water form on the metal...that ruins the browning surface.
The bulbs are set in ceramic fixtures, and the bottom compartment is lined with asbestos sheeting.
I have my .60 cal barrel, tang and rear sight in mine right now, and so far, it's turning out great. Just applied my last coat of solution...barrel comes out in three hours for carding, flushing with boiling water, and oiling. Should be able to sight it in this week...yipppppeeeeeee!