The phrase "plain cardboard" refers to cardboard that is not colored, decorated, or fancy. It is just a simple, regular, and ordinary cardboard material. Full definition
We start by looking at the package, which by no means is your ordinary plain cardboard box model kit, these transparent packages are pretty as they can be. (seganerds.com)
Make sure it's made of a safe material such as plain cardboard without heavy ink or a plastic finish and remove any tape or staples. (bunnyproof.com)
Cover plain cardboard box files with wallpaper or fabric for a fabulous bespoke look. (idealhome.co.uk)