Since the composite panels are made using the vacuum bagging technique that uses a flat table (Panels A and C) or by the Holt-melt process (Panels B) – they cannot be manufactured with a camber by way of insertions.
However it is possible- once the roof has been securely installed – to carefully put temporary props (eg cardboard tubes) inside down the centre line thereby pushing the roof up say 20-40 mm before the inside walls and cabinetry are installed – this creates a small camber.
Alternatively, if you are building an open span trailer – eg for cars etc – it is feasible to ‘roll’ some aluminium rhs and attach these to the inside ceiling to create a small camber. They can be attached to the DIY panel joiners if you are using them.