Your back doesn't have the Graflok retaining clips, so you need a slide-in type rollholder.

Sinar (Panorama, Vario, Zoom, Zoom2) and Calumet/Cambo (C2N) have made slide-in backs in 6x12 format. They're quite big and heavy, and you may find that the rear standard of an ultralight camera like the Ikeda flexes a bit under the weight.

The Sinar ones were frightfully expensive - the last new price was in the thousands of dollars - but now can occasionally be found used for a semi-reasonable price. The Calumet/Cambo is still listed new at $869.99.