Still working on that one Geoff.
At this stage I'm going for the simplest solution, using hold down straps on the inside of the trusses down to the wall studs. However on the two free ends of the each rail I will weld some of the excess rail I have into a "U" shape so that when the roof is fully closed of fully open at least one roller on each side will have a length of rail slotted through it, lined up and sitting in the groove.