Wayne,
This model will require a lot of work to make it resemble RPO/Baggage Car No 54 when in service on the Shavano and San Juan trains. If you can get copies of these two books, they will help answer a lot of questions.
Rio Grande Narrow Gauge Varnish by Herbert Danneman, published by the Colorado Railroad Musuem as Colorado Rail Annual No.25
Narrow Gauge Pictorial Passenger Cars of the D&RGW Volume II by Robert Grandt (Editor)
The D&RGW had both long and short RPOs which went through extensive changes over the years, including changes in the body lengths. End doors and end platforms were eliminated from from most RPOs as Postal Regulations changed restricting how the interior of the cars could be accessed and increasing the requirements for the safety of postal employees. RPO 54 ended up with no end platforms or end doors and steel rails running vertically from the bottom of the frame to the top of the frame on car ends to hardern the car against impacts.
The model in your photo would be a pretty good match for 053 in either the red or gray paint with the addtion of end ladders, hand railing along the top and roof walk boards on the left side low roof.
There are some good photos in Narrow Gauge Pictorial, Volume X, Numbered Work Cars of the D&RGW. These photoos would show other details that may need to be addressed on the model, depending on the amount of work you want to do.
Dave Adams