(Taken from 'Great Garden Birds' book by Sasol)
As far as nest-boxes are concerned, a Spotted Eagle owl nest box is easy to make. It is a deep tray - approx 600mm long by 450 mm wide and 270mm deep. Add a roof if you will erect the box in a more exposed positition (such as on the side of a building).
1. To construct a roof: screw wooden pillars (50 x 50mm) into the inside coreners of the box. For a flat roof, make the pillars 460mm long; for a sloping roof make the front pillars 460mm and the back pillars 360mm long. Cut the tops of the pillars at the correct angle for the slope of the roof. The roof itself should be approx 750mm x 600mm (five-ply plywood works well), so that it overhangs the tray on all sides. Place the roof on top of the pillars and attach by screwing through the roof into the pillars. With a sloping roof. it is an idea to reduce the height of the box at the entrance to 170mm deep. Remember that the front or entrance should be the width (450mm) rather than the length side.
2. Waterproof box with varnish or wood-sealant. Avoid anything that might be toxic to birds! If necessary, seek advice from your hardware store or paint manufacturer.
3. Erect the box so that the entrance faces away from the direction that your wet and windy weather comes from, and ensure that it is securely mounted on the wall or tree. Add a 10-cm layer of clean river sand to the bottom.
Alternatively, if you have a spare pot plant container, why not try get your own 'Pot Plant Owl' chicks?