I've been working on a number of things getting ready for the NE Proto Meet so I'm a little behind on posts. In addition to the modifications for the 600-series coaches, I've been working on an RPO.
A year or two ago Belthlehem Car Works released a NH Baggage car with 15' RPO apartment. Joe Smith detailed how he scratchbuilt a car with a 30' apartment on his blog.
The kit has a cast resin roof, injection molded ends, and 3D printed resin sides, with the original CAD by John Sheridan. The floor is a piece of styrene with injection molded fishbelly center sills. It's a pretty easy kit to assemble, although I felt the instructions could have been a little clearer in regards to building the floor. I'm not sure my approach matched what was designed, but it worked for me.
Here's a picture of one of the cars in Danbury. One of these cars was regularly assigned to train 131/136 so it's a key car for my layout.