I'm not a fan of the 21 bar harps. Too much clatter and hum for my taste. I like the 15 bar harps set up with any chord plan you like. I actually find it more better to have three harps to tune and siund the way I like than to have a bunch of chords I don't use soaking up vibrations and stealing sound.
Another plusto the 15 bar ( or less) harps is the added playing surface.
There are a number of blues auto harp articles about. Some I recall in the Auto Harp Quarterly about twelve years ago,