Security: you can't make it impossible for someone to steal it, you can only make it more difficult.
Wood vs. metal: Neither should be designed to contain an explosion. A metal box should be designed to come apart at the seams, reducing the number of pieces of flying metal. Wood becomes flying splinters, but minimizes condensation contamination and insulates against temperature changes better. Best answer: both - metal box lined with wood.
Static electricity: Hawg's right, unless the powder is contaminated with some resistive material that would heat up when exposed to static electricity.