I experimented to get this kind of an effect on a job app, and my reference point were some of the voiceovers in DotA 2 -- I love how well some of them are tuned to sound like they're coming from within a suit of armor. Here's a great example: http://www.youtube.com/watch?v=Yj-F3aI6m-g
Closest I got to this effect, after some experimentation (including a failed worldizing attempt through an overturned stock pot) was to use a clanging pot sample as an impulse response. I treated my voice with a little EQ, Compression, and some envelope-following stuff in Filter Freak before sending it through.
Here's how it turned out (you're interested in the 3rd / 5th sounds), plus some extra experimentation with a vocoder and using a Massive patch as modulator. I had a pretty tough time maintaining intelligibility without having the original, unmodified voice peek through.
Actually that reminds me -- intelligibility is KEY with V.O. Whatever you do to make it sound real, make sure that you can still understand what's being said above all else.