Malcolm,
The Atlas Berwick Hy-cube boxcar has an X-panel roof, although it is essentially flat without a ridge. I don't know if an X-panel caboose roof is the same as the X-panel boxcar roof, so I can't say whether a roof from that car could be adapted to your needs.
Also, there are drawings of the Stanray diagonal and X-panel roofs in the 1974 Car and Locomotive Cyclopedia. Drawings give panel dimensions, rib heights, and various edge and closure detail conditions. I could probably get you a scan of the drawings if you needed it for future projects.
Jim