I was able to do this with one of my wife's sets of fake braces, which didn't have headgear tubes. Basically, I used a wire cutter to clip the inner bows on the facebow, then I curved the ends into a "U" shape with needle-nose pliers. She'd wear it by placing the "U" behind the upper canine brackets, between the archwire and teeth. Attach a strap (we usually used cervical, but sometimes combi) and it's surprisingly sturdy and secure. Just be careful not to clip the inner bow too short.

Wish I could post some pics. Message me your email address if you'd like me to send you a picture.