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One of the easiest of paper models to make are shipping containers. Dozens of sites have free ones, and dozens more sites paid ones. They are not very durable but are super cheap using your computer printer. I brace mine on the interior with basswood beams so I can handle them and move them around many times without damage.
simply google 'HO scale paper shipping containers" or other scale and hundreds of singles will appear. If you leave out the scale (or the word model) you will get billions of hits on all sorts of paper boxes, etc. You can find blank templates for making your own 'fantasy' containers.