Measure the height and width of the vent. Using a table saw, cut a piece of hardboard three inches longer in both directions. For example: if your vent measures 12-by-18-inches, cut the hardboard 15-by-21-inches.
Miter all ends of the poplar pieces on a miter saw making sure to not lose any length. Lay them flat to miter them. Run a bead of construction sealant down the back of all four pieces of poplar. Place the long pieces of poplar on the long sides and the short pieces of poplar on the short sides -- glue sides down. Bring the mitered corners together. Shoot staples around the perimeter every six inches.
Drill holes around the perimeter of the frame using a cordless screw gun and 1/4-inch bit. Space them six inches apart, one inch from the edge. Place the frame over the vent. Insert 2-inch screws in the holes. Use the screw gun to tighten the screws, securing the frame over the vent.