My cushion is starting to go on the Markus also, any chance you can link me to the replacement cushion you bought?
Bloody VPN, it disconnected me when I pressed post and lost my original reply. Slightly shorter response:
Of course. I originally used the advise in https://www.overclockers.co.uk/forums/threads/modified-ikea-markus.18876340/. There's a reddit link in the thread that contains the OP's full instructions.
I used the same website (efoam uk) but instead of the rectangular foam I ordered this:
Rounded edge cushion: width 53cm, length 52cm, thickness 7cm (Reflex Superior (firm), Stockinette undercover)
I still had to contour it a bit using a bread knife.