I have not seen the S5 boot shape however the Coupe had a surprisingly practical shape but with the awkward opening. I used 5 bags and stood them in different ways.

https://www.amazon.co.uk/Lifewit-Oversiz...Holdall+Weekend

5 of these and some smaller bags (camera, snacks, handbag etc)

Everyone loves a Morgan. Even me, unless it's broken again.