A sheet of Lucite forms the window. Temporarily place waxed paper behind the hole and support it with a wide scrap of wood. Then run a bead of silicone cement around the Lucite and press it into the sill so it is fiush with the wood. Make the seal airtight by caulking the joint with silicone cement. To bolster the tube, fashion three tight-fitting rectangular frames from wood slats and glue them to the outside of the tunnel.
Horizontal slats help to make the structure more rigid and support the measuring equipment.
Some household fans can drive wind as quickly as five meters per second. Decreasing the cross-sectional area in the test region by two thirds will, for example, nearly triple the airspeed you can attain. If you need still stiffer winds, check your local industrial liquidators for more powerful fans. Most commercial fans use a four-position switch to control their speed. For finer adjustment, replace this unit with a household light dimmer.