A couple of MTH offerings:
MTH Steel Sided Cupola Caboose
A fantasy I'm afraid. The CNW had many Woodside Cupola Caboose. I can't find any evidence of a steel sided one. The details are nice and an interior with figure too.
Oh, to lay my hands in an accurate brass one at a reasonable price.
I have the book CNW FINAL FREIGHT CAR ROSTER from the CNWHS. Northwestern had only Bay window cabooses at the time of merger. The only cupola cabooses came from the Rock Island or the CGW, which were used in MOW trains.