1. Place a rail and one Block in front of the rail and another one on top of the first block as reference.
2. Make a Template of these three blocks, using the button "Copy" in the advanced build mode panel
3. Press "Paste" in the advanced build mode panel.
4. Rotate the preview using mouse wheel or Left Strg+mouse wheel.
5. Pasting the preview.
Preview and pasted structure are identical.