Replacing leaking rooflight

'm replacing an MPK 280x280 rooflight in my shower room in a Renault Master 2004.

The sealant/mastic tape looks as though it's seen better days and, because the rooflight is a few years old and looking a bit brittle, I've bought a replacement for 30-odd quid.

I'm getting confused by the number of different opinions - and youtube videos - on how to install this properly. It's a like-for-like replacement and so just needs 'sandwiched' over the existing hole by screwing together the top and bottom sections.

Some people say to infill the corrugated sections on the roof (roughly 7mm drop in height?) with plastic/Perspex, stuck in place with Sikaflex (or similar). Others say use butyl tape in layers - some use mastic (W4 or similar).

As far as sealant/adhesive goes, there's a wide range of opinion. Some saying Sikaflex/stixall do the job. Others saying these are too strong and will be very difficult to remove later. I do have some PU sealant from a motor factor that I've used to seal seitz windows on a previous van so this is another option.

Basically, I'd be grateful for clear advice on 1) dealing with the corrugated roof sections and 2) what sealants adhesives to use and how.