I got all of my matching Tachihara Hope yatsugiri holders bundled with Rittreck bodies.
Eastman-style WP holders will fit the Rittreck back. The image area will be a few millimeters off center in the long dimension, but you could always mark the ground glass if you're trying to be that precise. The Eastman holders can also be a bit hard to find, but don't forget that Eastman plate holders with film sheaths can also be quite workable.
The reverse is not true, BTW - the end-to-locking-ridge dimension on the yatsugiri holders is just a bit too long to seat properly in a back designed for the Eastman holders.
Current Chamonix WP holders should fit as well - I can dig out a Rittreck WP back and check that with mine if you think you might want to go that route.
Good luck!