I made my own bowed psaltery in school when I was 16. I thereafter made a further 5 or 6 for other people including the Corries.
You can only play one note at a time, dodging over to the other side of the box to reach the sharps and flats. For a longer note, use a longer bow. I followed Bob Common's instructions on that, rather than those in Musical Instruments Made To Be Played, from where I got the plan for the instrument. Bob Common was a Yetti. He made the psaltery played by Bonny Sartin.