I’m using a single 4 inch telescope mirror here. Setup is not particularly hard but is a little tricky mostly in terms of getting the alignment just right. There are also 2 mirror setups that can be used. Here’s a picture of my mirror mounted behind my printer.
Here’s a quick link describing the technique in more detail.
Here’s another example I did using a simple power resistor as the heat source:
I’ve been wanting to try this for awhile on a 3D print in progress. Getting a good angle is a little tough because the camera has to be pointed right at the mirror in order to pick up the differences in refractive index present in the air being stirred around.
Sometimes you can get a good deal on ebay for a telescope mirror, which is where I got this one. At 4 inches, it’s a bit small but still allows the principal of Schlieren photography to be demonstrated.