this is what i do, and have done many times with wardrobes etc
what width is it, do you want a door the full height
you maybe able to get a standard internal door same as the one on left
and then infill the top, which can be wallpaper or painted to match room
Still have the height to the top, just add a shelf about 1/2 / 2/3rd in
can you still get the picture frame moulding to match and bring that round
a bit of an internal frame and architrave and normal hinges
TBH I'd just infill with a timber stud frame having an opening to take a standard 6ft 6in x 2ft 6in or 2ft 9in door, board the frame with PB, install a wooden door casing and swing the door and fit architraves. You need a frame of some description to carry a door in any case and you also need to position the door so that it misses the moiuldings on the RH wall, so you aren;t realistically going to get an 8 x 4ft door in there
It would look weird with a massive door beside the other original.
Better to try to match the picture rail and wall finishes to either use a reclaimed door or use a solid ply door/doors made from sheet and painted the same as the walls? Use magnetic catches that are spring loaded for a seamless effect?