I also wanted to fit a sunroof to my last Multi. I took it to a reputable fitter who looked it up on a list he had to work out the correct size - but sadly, his list showed that the standard Multi was unsuitable for a sunroof as the roof panel would be too weak if pierced.
If you look at a non-sunroof Multi beside a FIAT fitted twin sunroof Multi, you can see that they have completely different roof panels.