I think I have a final solution for sliding roof...
Bunnings slide door guide (I will need 3 pcs a 2.9m to have 2x4.5m) and
wheels (from ebay), 8 of them, rolling inside on "L" 20mm profile.
The frame will be standard treated pine 70x39 timber, covered with colorbond.
Coming weekend I plan to remove the rest of the ceiling, to expose the roof structure and see how to proceed from there.