Neatly cut a single piece of 1/2" CD exterior grad plywood that is sized to fit snugly inside the door opening against the doorstop. Consider installing 2" x 4" blocking within opening for back of plywood to rest against snugly.
Cut an opening centered and 6" below the top of the plywood and install a metal soffit vent that covers the opening but allow light to enter the structure once the plywood has been installed. Use screws to attach the vent to the plywood.
Cut a least two 2" x 4" wood support members that are 16" wider than the door opening.
The support members are to be mounted horizontally and flush against the interior door casing with 8" extending left and right of the door opening. The top support should be located within 1/4 and 1/3 of the door opening height from the top of the opening. The bottom support should be located within 1/4 to 1/3 of the door opening height from the bottom of the opening.
Drill at least two holes in each of the 2" x 4" support member and the plywood. At each hole, insert a 3/8" diameter carriage bolt—from exterior to interior—through the plywood, through the door opening and through the 2" x 4".
Insert a washer over the end of a 3/8" diameter round, smooth head, carriage.
Line up each hole in the 2" x 4" support member and the plywood. At each hole, insert a 3/8" diameter carriage bolt—from exterior to interior—through the door opening and through the 2" x 4".
Slip a 3/8" diameter nut and washer over the end of the carriage bolt inside the structure and securely tighten the nut - pulling the 2" x 4" and plywood toward each other until there is no play.
Prime the exterior surface of the plywood.
On the exterior, caulk the perimeter edges of the plywood.
Paint the exterior surface of the plywood a neutral color or a color similar to door casing.