A rail on each side for rollers like a garage door or a slide system. Unistrut comes to mind for rails that will be stiff enough when supported on the far back end with a post when it is opened. Build the top of corrugated fiberglass barn roof panels or corrugated steel framed with 1x4s.
I'd take ideas from the awning makers. Since you can weld, I'd use conduits for frame, heavy duty plastic tarp (uv resistant) stretched tight and a tackle to lift the whole thing and set aside for filling. You can frame out a rail as well for sliding the roof out of the way. If you have a snow load, you'll have to plan for sweeping the snow off/breaking ice from inside. I think metal roof is way too heavy duty for this application.