I bought a boot board fitting kit from Mini Spares then cut my own board and covered it in acoustic carpet. I also covered the inside of the rear seat back, inner wheel arch and quarter panel.
It looked really clean although I’d probably add a courtesy light if I did it again as all the black really sucks up the light especially in photos.
I think the piece of upholstered card that covers the back of the bootlid looks rather neat too. quite a clean and simple design and might hide any sound deadening material that was fitted inside the bootlid.