Fascinating info, I've looked at DYI headtracking before (although I do have a TIR5). Often just to realize I probably could've done it myself, just being lazy

To me, it seems the camera is the big expense; though I've read that PS2 Eye cam IIRC?) can work. You might find those used for cheap. As for the actual TrackClip Pro LED array, you can buy those on eBay, brand new from a US seller with 99.9% positive feedback and FREE shipping, with 30-day returns as well for only $38 (no, I am not affiliated with this person or auction listing in any way at all, just noted it myself once when looking at replacing a broken unit). Seems fairly hard to beat, when you account for your time (although I do sincerely appreciate doing it as inexpensive as possible, and when money's tight, that means your time doesn't count lol)