The roof of the bus is fibreglass and has a 45 degree metal plate at the edge with an air space behind it for cabling. The rest of the roof is plastic sheet with a metal bar running the length of the bus.
What I am planning to do is remove all the metal plates, and replace them with plywood sheets Velcrod into place. I am looking at this material to line all the roof and a white/off white material for the panels. I think this will look good and will allow me quick access to the wiring should I need it.
I would ideally like to use a spray foam to insulate this space but getting it in without clogging up the wiring will be hard so I will be using a combination of kingspan and normal loft insulation.