If you have a decently modern phone you can get the smoothtrack app on your phone for $10. You can use that as an input for opentrack. I've been using that for a while, and it seems to be working well. Apparently you can also use a regular webcam in opentrack using aitrack, but I haven't personally used it so I can't vouch for how well it works. Source: 12 months ago

For the last 6 months, I’ve been using a app on my phone for head tracking. It’s called https://smoothtrack.app/ and it’s only $9.99USD links to any front facing camera and works very well. I originally bought it because the trackIR pro was on backorder but since using it, I feel no need to change. I have no affiliation with this app.I’m just very impressed by how simple and effective it is. Source: about 1 year ago

Yeah, it works with opentrack. Pretty seamless. You do have to download some connection software to get them to talk, but that’s all covered on the SmoothTrack site. Source: about 1 year ago

Smoothtrack costs $9.99 and only needs a smartphone. Surprisingly reliable and consistent if set up properly. Source: over 1 year ago

The app's webpage, scroll down to the setup section for the usage guide. Source: over 1 year ago

There's also https://eviacam.crea-si.com/ which is a great accessibility tool and can sort of do the same thing. Source: 12 months ago

I did some quick search and found something. This is not what I had seen previously but seems close enough: https://eviacam.crea-si.com/. Source: about 2 years ago