See here for a discussion the outfitting of a nyckelharpa case for the harpa.
Some cases leave room for an accessories box long enough to hold a short bow.
But it bounces around in the box. It is not hard to mount the bow on the inside of the case cover, again like violin bows. Because the bow will be much shorter than the case, standard violin bow spinners may not be effective at constraining a NH bow, though a pair of spinners may work. With foam pads at the ends of the inside of the case cover, I found it easy enough to carve out a small pocket of one and stick the tip of the bow into it. If you don't use pads, a small pocket for the bow tip can be fashioned out of some fabric stiffened with interfacing and fastened to the inside of the box.