I can recommend the Horseman 6x12 roll film holder which I use with my Ebony 45SU. It is rather on the expensive side, presumably because there is a more limited/specialised market for the product, but a very good product.
The Horseman 6x12 RFH's are going for cheap on the disgusting auction site. In fact, there's a glut of them right this very minute. You might get a really good deal!!
Cambo also made some roll film backs. 6 x 7 6 x 9 and 6 x 12. The 6 x 12 fetches a far price. The 6 x 9 is normally quiet a good buy and the 6 x 7 is very common. These fit any model with a universal back.
Pat