There's definitely a Rollex back to fit the Ideal, you'd need to match the plate holder edge style though. I have at least 6 Rollex backs all of slightly different 9x12 fit, plus 3 Rada ( later taken over by Linhof).

I have some data on the Zeiss plate holder edges/fit somewhere I'll try and find it this evening.

9x12 cameras and accessories like roll film backs are easy to find if your patient but you're in the best country to find them I always search ebay.de

Ian