You could start with this MDC car:
but use the version with the the tongue and groove sides, and a roof like this car:
These cars are about 56' long over the platforms, so they'd need to be shortened. However, before you do that, cut a block of wood to fit inside the car, then run it lengthwise (down the middle, with the roof in place) through a table saw to bring the width down to what it should be. Glue the halves back together, then cut a section out of the length to make it shorter. After you glue it back together, install a cupola. The old Silver Streak Southern Pacific style is pretty close, but it's metal. An easier-to-find option would be the Walthers SP C-30-1 or the PRR N6B, and since these are plastic, easier to make narrower.
For the radial roof car, scratchbuild the body from styrene. You could also use styrene for the roof, or use the wood one from Northeastern. Check the Walthers catalogue for detail parts: New England Rail Services have windows that could be modified, and Grandt Line offers parts, including doors and windows, for narrow gauge cars.
Other than the basic body shells, you could parts-build these cars right out of the catalogue.
:-D
Wayne