Hiya bud, If you get a piece of light ply or chipboard, something a little stronger than hardboard, say about three or four hundred mill wide and as wide as your bus. Clamp it somehow to the cant rails or similar in the position that you want to measure the curve. Make sure the bottom edge is level and equidistant from the cant rails, by level, I mean level inside the bus and not spirit level type level. On this piece of scrap, draw lines square to the bottom edge at approximately every 20mm. Now make yourself a marker post just a little longer by a couple of inches than the biggest gap, this post needs to be something like a pencil size but with a square end at each end. Start in the middle and hold the marker post parallel to the squared marks on your new template, the squared lines are just there to give you a guide to holding your marker post in a square position to the bottom edge. Hold the post lightly to the roof of the van and mark with a sharp pencil at the bottom of you marker post. If you do this carefully on each line, you will transfer the roof shape to your new template in small dashes, join them up then cut the template and try for a fit. This will work and will be a replica of your bus roof and not anyone elses. Bear in mind that the roof shape changes dramatically the more you move backwards and forwards, you would also need to make sure that your fitted template is parallel to the rear of the bus and vertical to the bus, again vertical in the bus and not to a spirit level.
Ozziedog,,,,,,,,,,,,,,,,, Easy innit :lol: