I can't be of much help on anything aside from the harp... But I have to tell you, to make a real working harp is probably more trouble than it's worth (and technically you'd be trying to make a lyre, not a harp). It's a helluva lot of work, and in a convention setting, rather pointless to have since you wouldn't be able to hear it. It also depends on the quality you're going for - are you making the equivalent of a shoebox guitar or shooting for a quality instrument?
While I've yet to see a real harp or lyre in the shape of the Goddess Harp, I did find this blog on how to make a real harp
. If you still want to try go ahead, though I'm not sure how wide a variety of sound you can get with a true Goddess harp styled lyre.
Just remember, it's probably very possible, with tech nowadays, to make a prop harp and put some sort of sound device in there to make it sound as if it is being played.