I have a bunch of New Bright stuff and had an extra caboose but really wanted something that I would suppose would be used as a short passenger. I didn't want to spend much (time or money) partly because it is New Bright stuff and just for the challenge. I didn't get too far before a few bucks were spent on a balsa block (for internal roof support) but did save time and frustration. Then I had to buy a new can of spray paint because the two partly used cans became clogged. Superglue was needed and then masking tape. Nuts. Ok, I spent more than I wanted but it was still cheaper than trying to locate a passenger car. I did use some stuff from the recycling bin -- plastic food container for windows (to match the New Bright windows on another car), paperboard for the roof, fishing line, and even a brown paper bag. It did take quite a long time given the cold winter here -- painting inside has its downside with the fumes. Anyway, it turned out well enough. I still have that Toy State car to get G-gauge wheels for.
Thanks. I also learned to use sanding sealer (many applications with sanding of course) to make a smooth paper roof and to stiffen it. Gluing multiple layers of paperboard makes a stiff product also.