Two six-foot 2x4’s
One sheet of ¼ inch plywood (in our case it was 44x72 inches)
2 yards of black felt, 2 feet across (or fit to window)
Wooden molding (½ in.)
Step 1: Making the frame
Cut a 2ft. portion from one of the 2x4’s for the base of the heater
Cut 4 inches from the other 2x4 then cut in half (keeping in mind that 2 inches on each side of the frame will be comprised of the 2x4 laying across the bottom of the frame)
Step 2: Adding plywood to the frame
Nail the 2ft. 2x4 into ¾ of a rectangle at the top of the two 68in. 2x4’s at right angles.
Place glue around the frame on one side and nail the 72x20in. plywood onto the 6x2ft. frame.
Step 3: Add the Felt
Next, use the drill to drill holes through the felt and plywood. Any size and number is acceptable, keeping in mind that this is to allow air to circulate upward against the black felt
Use scissors to clean up edges around the holes to allow optimal air circulation.