I have several of these ceiling vents in my apartment. The ones in the bedrooms blow right onto the bed and it's annoying. I did not want to get one of those big and ugly deflectors, and the magnetic sheets would not work well because the grill slats are raised up about 6mm. So I made this. It partially blocks half the vent but there's a little space between the cover and the vent. This prevents backpressure. I designed this to work with 11mm disc magnets. The insets are 12.5mm x 1.5mm. The 11mm disc magnets are 1.7mm thick, so it's a nice fit. I used super-glue to hold them in place.

Total width is 177mm. The vent cover portion is 155mm wide.