Set a metal sheet face-down on a flat work surface. Place the wood frame on top of the metal sheet. Measure out from the frame so that the metal will wrap all the way around the back side of the frame with 1 inch of overlap. Draw this measurement with a permanent marker. Repeat for the inside of the frame.
Cut the sheet with metal shears along the marks.
Run a bead of polyurethane-based adhesive on the front of the window frame. Put the frame down on the metal sheet. Attach the metal to the frame with clamps. Let the adhesive dry overnight.
Cut the metal with shears from one corner of the metal sheet towards the edge of the frame. Repeat at each corner of the frame.
Hold one edge of the metal with pliers. Wrap the metal sheet around the wooden frame. Hammer nails through the metal and into the wood. Bend the metal sheet around the edge of the frame.