Does you box make some noises? It may sound funny, but mine is in the same place and actually makes some noise. I have no idea if it is normal or not, but given the chance to do it over (and I will), I'll put it inside the engine compartment.
With all respect to Grumpy, vibration you'll have inside or outside. Moisture should not be in issue under the hood up high (watch it when cleaning the engine compartment). Heat is an issue or at least can be under the hood. Grumpy raises ideas of what you'll need to compensate for or try to minimize. Most mountings I have seen, however, are in the engine bay. I like the ideas of putting it in the cowl area...only if I wanted to hide it. Heck, I like the look of an MSD ignition!! It costs enough to display it
Davy