Make an exhaust shroud for a DIY air purifier

An exhaust shroud helps force more of the air from the box fan into the filter, improving filtration performance. At CHC we use die cutting for our cardboard shrouds, but they can be easily made at home with just a few materials. This shroud will fit a basic Lasko box fan (B20201). A one-page downloadable PDF of these instructions is available at this link.

Supplies

  • A piece of cardboard at least 19” square (we like to use the box the fan comes in)

  • Metal or wooden yardstick

  • (Optional) Smaller metal ruler

  • (Optional) Carpenter’s square

  • Two pencils / pens

  • Utility knife or razor knife (e.g. X-ACTO)

  • Cutting mat or piece of scrap cardboard

Instructions

  1. Trim the cardboard to 18 3/4” square

  2. Find the center of the square by drawing an “X” using the diagonals from the corners.

  3. Make a “quick and dirty” compass:

    1. Cut a strip of cardboard about 10” long.

    2. Mark two points 8” apart. This will be your compass’ radius.

    3. Poke a pen/pencil through each point such that the tip just barely breaks through the other side, and leave them in place. Now the compass is complete. (See 3.3)

  4. Draw lines for cutting:

    1. Take the compass and poke one pen/pencil through the surface at the center “X”. This is your pivot. Use the other pen/pencil to draw a whole circle. (See 4.1)

    2. Draw four slots with the dimensions shown, two along one edge and two along the opposite edge. (See 4.2)

    3. Mark a curve at each corner:

      1. Mark a point 3 1/4” in from both edges.

      2. Poke a pen/pencil into another spot on the compass so that it has a 3 1/4” radius now.

      3. Using the point you marked as the center, use the compass to draw a quarter circle. (See 4.3.3)

    4. Put the cutting mat / scrap cardboard underneath the main piece, and cut on all of the lines. If you’re using scrap cardboard as your cutting mat, take care to avoid cutting through it onto the table! Cut using three passes:

      1. A light pass into the surface layer of the cardboard

      2. A second pass that cuts into the middle corrugations

      3. A third pass that cuts all the way through. (See 5.3)