Not sure what kind of load you're going to have on that 5 watt resistor, but a few years ago I enclosed a 4 watt night light in a 4" x 6" x 8" wood box to protect an alarm circuit box from sub-zero temps out in a remote shed.
To my surprise it failed because it got too hot in the box. I couldn't tell exactly, because it pegged a 120 degree thermometer. I ended up leaving the lid about half open. That still gave me a 30 degree temp boost, which was all I needed.
Another project - right now I'm using a 1/2 watt 1K resistor in series with a 12V flashing LED connected to a 12 5AH battery. The battery lasts about 6 weeks with the resistor or less than 2 weeks w/o the resistor. Can't really tell any difference in brightness either way.
