Hi
That's a deep rabbit hole.
If this is just for fun and games, then whatever you want goes.
If you want a real aero engineer to take anything you measure seriously, you have a long shopping list of things to build.
There's a lot of science, technology, and history to the development of wind tunnels if you want to dive in. NACA published a ton of reports on how to do this well. Yes, in the 1920's and 1930's but it's still the same air the dinosaurs were breathing.
Turbulence is your enemy. Wind tunnels can easily magnify turbulence if you don't know how to avoid it. Many people making a home-made wind tunnel make a bunch of fans blow AT the test subject. This is wrong. That puts so much turbulence on the test subject that the results are always nonsense. Make the fans suck at the very end of the tunnel.
Walls are also a problem. It's hard to imagine a wind tunnel without walls, but the perfect tunnel wouldn't have any walls at all. Along each wall is a boundary layer of static air. You can make the walls smooth but the flow velocity along the wall is still zero. All you can really control is how thick the boundary layer is. A few inches or a few feet. It grows as you go farther through the tunnel.
If your test section is a meter across, the usable part of it is about 1/2 meter across. If you've catered to the turbulence carefully enough. Careful measurements of the flow velocity profile are needed to show that you have smooth flow.
The inlet can start you off on the wrong foot if it induces turbulence too. A plain open tube has an edge where air entering the tube has to shear away from air not entering the tube. To solve this, a "bellmouth" is added to the inlet.
The simplest home-made approach is to have a bank of fans as the exhaust. From the back, in upwind order, is the test section, flow straighteners, plenum, and inlet bellmouth.
With that for reference, see how much of this you really want to take on and decide how much effort is worth the results you want to get.
