I am trying to harmonize with a vocal box and nothing seems to work right. Is there a key that is close?
D would fit. Sometimes not only the chords from the parent key (D here) are in a song, but also those from its parallel key (D minor) feature. D G and A are from D major, and C and Bb come from D minor, which uses the notes D E F G A Bb and C, so CEG and BbDF become available.
HOWEVER you're NOT going to be able to use a harmonizer set at ONE key, as this is effectively switching from maj.to min. A harmonizer which is connected to a guitar, and can recognise the chord of the moment will do the job, but set yours to D and half will be good, set it to Dm and the other half will be good! Not really practical. Even if your song was, say, in D and had a simple modulation to A, using an E chord, the harmonizer would be unable to compensate without being told which chord that bit's on.
Most often, a piece will end, and sound like it's finished, on a particular chord. That's usually the key chord - it's 'come back home'.