A flat piece of metal about 1/2 inch wide with two 90 degree bends is most commonly referred to as a cornice bracket for ceiling mount. A standard angle iron, 1" x 1" or larger, can be used for...
www. creativecornicing . com has pre-mitred corner pieces ready for DIY applications. All you need is glue. Fit all the corners first, then simply fill in the spaces with the longer lengths.