Another alternative to consider https://z4-forum.com/forum/viewtopic.php?f=9&t=76562&hilit=bcs+backboxes
& i do believe John-e89 still has tucked away a one off pair / bespoke set of M backboxes he could be up for selling at reasonable money :wink: came off my last ZMR after 1500/2k miles pic here https://www.instagram.com/p/BtNvaggh_ty/
& a sound clip of them here https://www.instagram.com/p/Bmpxasglm16/