Some dimensions of clerestory roofs from the 1928 "Car Builders' Cyclopedia" and measurements of Rivarossi and Micro-Trains cars:
Standard Pullman Sleeper: overall width of clerestory roof = 5' - 11 3/8"; side overhang = 1 1/4" beyond deck wall (Figures 1227 & 1228 - overhang scaled)
Canadian National Sleeper: overall width of clerestory roof = 5' - 11 1/8"; side overhang = 2 7/8" beyond deck wall (Figure 1237)
Canadian Pacific Private Car: overall width of clerestory roof = 5' - 10 3/4"; side overhang = 1 1/4" beyond deck wall (Figure 1253)
Rivarossi sleeper: overall width of clerestory roof = 6' - 0 7/8" (actual measurement = 0.455")
Micro-Trains observation: overall width of clerestory = 5' - 11 3/8" (measured at vestibule end where there is no a/c ducting - actual measurement = 0.446")
Note that the M-T observation has a pedimented end at the vestibule end of the car and a plain horizontal lower edge to the roof at the observation end.
I don't have a parlour car to check the roof dimensions. "If" it has the a/c duct on one side only, it should be possible to cut two lengthwise and splice them together to give a plain roof with the pedimented shape at the ends.
With either the Rivarossi or the M-T roof, if you add the roof overhang of the prototype you end up with a roof that is marginally too wide.
Cheers,