@crazymilk
This will probably sound better planned than it really was but anyway
The roof had been ply lined before and was aweful as they had tried to secure it to the metal cross rails at the B,C & D posts with no bracing between. The problem with that is the roof has to bow in two directions, side to side
and front to back. Also with no bracing between these cross rails the ply was too flexible.
So the idea with the placement of my battens was to smooth out the front to back curve and add reinforcement between the cross rails. The battens are glued to the roof at each end but also have a piece of wood inserted in the middle to fill the gap between the batten (straight) and the roof (curved). This together with getting the ends of the batten to be pretty flush to the cross rails means the front to rear curve is minimal.
So why is that important? Well, if you get it pretty straight front to rear then you can pull the ply up into the roof with the fixings/screws into the cross rails and it will create the side to side curvature in a nice tidy bow as it is only bending in one axis. This in turn meant, by cutting the width to the right size
I didn't need any fixings on the side of the ply roof lining.
The ply is actually held up by 9 screws into the cross rails and the tension of the bow holds it tight against the side members of the roof
This was helped by getting the shape down the sides "just right", noting for example that it narrows as you go from the C to D pillars, but if you do this carefully it will stay put and be secure just with the screws into the roof bracing.
Hope that makes sense, any Q's let me know
Cheers,
MM