I built one from a kit for my wife a few years ago (non-cardboard). She hardly messes with it but the kids and I love to pick it up and tinker. I can hardly think of an instrument that rewards the least effort and care with such pleasant results. As said above, though, really mastering it -- like any instrument -- is a lifelong process.
As for the RA: there are two basic ways of holding the noter. One (I think the more 'orthodox' way) is to hold your left hand with the palm down and basically press down on the noter with your index finger. If that results in an uncomfortable wrist position, you could try the less conventional way of holding your left hand with the back of the thumb upward and pressing down on the noter with the thumb. This is probably a little less agile but may well result in a more ergonomic position for your wrist.
JMHO
-Glenn
(PS hammered dulcimer would be a poor choice both from a learnability and wrist abuse standpoint.)