You don't need a clip per se, but the current version of opentrack has the facetracker disabled. You can print and use the "Aruco" marker that gets mounted near you forehead and the camera will track it. I've only really used OT with the 3 pt clip, mine is homemade and works fine. Information for other tracker methods w/OT is sparse too say the least. Try a google/youtube search for opentrack/aruco/headtracker and you should find something to help you out. Also, browse through the wiki too get some more information.