Spotted this window treatment in Living Etc's March issue - it's easily achieved using our plain window film or you can order the exact size made to measure.

Window film is easy to apply.  In this case I would use plain film by the metre. Measure the window and cut out the film but shorten the length by 4 or 5cm. Then cut the film across where you want the gap to be, i.e. about 20 cm down from the top so you have 2 pieces of film to fit. Apply the film to top and bottom sections of the window leaving the clear gap inbetween. Ideal for seeing out but not easy to see in.