We installed tall IKEA wardrobes in our house and this ceiling vent is nearly entirely covered. Rather than having HVAC treated air directed towards the back of the wardrobe and wall, I created this deflector that directs air towards forward. 

A simple model, I printed this with PLA, though I may replace it with a more durable filament in time.

Supports are required if you print as oriented. It thin and shallow, but works well for this low profile diffuser. Inner diameter is approximately 6 inches, and outer diameter is about 8.5 inches.