When you have a duct opening in the toe kick area of a cabinet it is important to measure accurately to make sure you order something that will fit right. You should not only measure the dimensions of the duct opening, but also the distance from the floor up to the underside of the cabinet. Some people have a lot of space there, and others have a minimal amount.
Our narrowest toe kick vent covers are the Rickenbacker style and the opera grille, both of which are 2 7/8 inches wide. The quick ship slotted and one piece vent covers are both 3 3/4 inches wide.
For more choices in which the width is variable and easily made to your specifications there is the turn of the century collection (metal) and geometric vent covers (wood).