For those flags that cannot be displayed at half-staff, such as a flag that is hung flat against the wall, there is a tradition that is not mentioned in the U.S. Code that provides for the use of black ribbons as a sign of mourning.[1]  Two streamers of suitable length (see last paragraph) made of black crepe or ribbon should be attached at the top of the pole just below the finial or ornament at the top of the pole.  For a flag that is displayed flat against a wall either vertically or horizontally, affix a streamer at each end of the top edge of the flag.  Do not, however, fasten the streamers to the flag.

There are no formal specifications as to the length and width of the black streamers used to convey that the flag is in mourning when the flag is fixed on a pole, but the suggestion is that streamers could appropriately be from about one to three inches wide with a length approximately equal to the hoist (smaller side) of the flag.