My wife went back to school and she needed a place to study so I made her an office in the cupboard under the stairs (literally). The problem was it was cold in there but there was an HVAC line pipe running through it.
Cut a hole in your vent the size of the outside of the flange (57mm or 2 1/4"), insert the flange and screw it in place. Then add the control (the tolerances are TIGHT, so you might need to add a little lubrication).
Turn one way and you get a little hot/cold air; turn the other and it's closed.