The back inside of the faceplate piece contains strong magnets also. This holds it in place with the opposing magnets that are contained within the portion that's installed into the wall.
These custom made grilles are made to order in just the size you need. The above is shown in the Como Block pattern. Constructed of Baltic Birch 1/4 inch hardwood ply, choose veneer of oak, cherry, maple, or paint grade wood. $1.75 per square inch of duct opening. Ships sanded, ready to paint or stain.