Product review (5 stars): Works on ceiling vents
I bought these from my local Menards to cover the vent in my office at work because when they have the heat on, it heats my little office like a sauna.  I placed these on my ceiling vent 3 weeks ago and they are still sticking, even against gravity!  I just wish they came in bigger sizes to cover the whole vent.  2'x2' would be perfect.