We wanted to keep the van looking as much like a normal van as possible from the outside so we decided to only have a skylight and window at the front.  I was a bit nervous about cutting a big hole in the fibreglass roof to begin with, but once I got started it was fine.  Its all stuck down using strong sikaflex adhesive.  The window frame was made really simply with some battens and some polycarbonate sheeting cut to size.