If you are in the market for a DIY wind tunnel the folks over at sciencebuddies.org have got you covered. They have done a great job documenting how they built their own wind tunnel. Most of the structure is made of plywood with the test chamber is made of plexi-glass so that the operator can visually observe what is happening during a test. A common gable-mount fan provides the air flow, you may have one installed in your attic to keep it cool. The only non-widely available components are the force sensors that feed data to a computer for logging.
This DIY wind tunnel works the same way as most open-circuit wind tunnels do. Air enters the Contraction Cone, where the inlet is larger than the outlet. This reduction in chamber size speeds up the air as it enters the tunnel. The air then passes through a Settling Chamber that straightens out any turbulent airflow by passing the air through a screen or mesh. Up next is the Test Section where the model is mounted on force sensors. The sensors in this particular wind tunnel are set up to measure the force resulting from the air pushing rearward on the model (think drag) and also how much lift is generated from the geometry of the model (in this case an airfoil). By the time the air reaches this point it is moving straight and parallel with the tunnel which avoids unwanted forces from turbulent air applied to the model. Ideally, all forces applied to the model will be a direct result of the aerodynamic properties of the model. Any wind speed sensors would also be placed in this section. Rearward of the Test Section is a Diffuser which is a chamber that slows down the air by gradually increasing in cross-sectional size. The final portion of the wind tunnel is the Drive Section that contains the fan. Having the fan at the end of the tunnel almost sounds counter-intuitive but doing so reduces turbulence of the incoming air.