Not that easy, me thinks
I had the following problems with a bus with the standard VW sunroof hole;
standard 8'x4' isn't quite wide enough, so you have to turn it 90 degrees, which means the grain runs the wrong way - across the width rather than along the length of the van, which IMO isn't asthetically pleasing. Also this means it doesn't want to bend in the direction you need it to, so fitting is a pain (but it can be done)
Even turned this way the rear section isn't quite long enough, so you either have to hide a join in your roof cupboard, miss a bit off at the rear (it's hidden by the cupboard anyway) or extend the two pieces that run along the side of the sunroof to meet the rear panel (but then the alloy extrusions that join the panels will be slightly too short).
I went for the third option, but decided i really didn't like the grain running in that direction. So i ripped it out and bought some 3mm cream plastic from Centry plastics in Sheffield/Rotherham (Waleswood ind est). This fits so easily (bends in all directions), can be cut with a stanley knife, looks good, is wipe clean and brightens the van interior. I love it 8)
They do loads of different colours and thicknesses to suit many an interior fitting problem
http://www.centuryplastics.co.uk/centuryweborders/
Hope that helps
popcorn