Steve,
It's been my experiance that for true concealed carry a small to mid frame revolver is easiest to coneal. Something like a S&W 66 in 2.75" barrel and round butt or a smaller J frame. I have one of those and carried it very unobtrusively just in the waist band of my pants back by the rear kidney area, strongside. It was far enough back that I could get to it with either hand and easily covered by a shirt tail or untucked T-shirt. Yep it's 6 (or 5 depending on model) rounds and reloads are easiest concealed loose in the pocket.
I like the locked box idea, but am not sure about the lid locking mechanism and would want a heavier cable to secure the box to the vehicle.