If you are going directly to the roof skin you'll need a top end glue that will stand 100C . Normal automotive spec is 50c and heavy duty is 70c . I found a place down in Bournemouth, Hurn Airport as was, think it was on the service road that can supply it. Plus there was a company Worthing way , Parafix I think, that could supply a 3m spray glue to the 100C spec.