This is a sliding air vent deflector that supports widths from ~18cm to 35cm. The left and right edges include a lip that can sit under the vent to help hold the deflector in place. I printed it in PETG for extra heat tolerance but PLA might also work depending on your heat output.

It holds together when placed on a surface because the larger piece grabs the front edge of the smaller piece while also wrapping / bracing it at the back. See the attached video.