Geometric vent covers
These are a great choice not only for their styling and 26 different designs, but also because you can specify both the overall dimensions and the design opening dimensions. This is helpful for those who have a difficult situation where they may not have room for a flange on one side, or some similar problem.
Construction is of 1/4 inch high density plywood, they have a top veneer of either oak, cherry, maple or paint grade wood. They are provided unfinished. If you need dampers be sure and review the page that discusses that since dampers are implemented differently than they are with other types of vent covers.