We clamped the metal down really well so we could cut the corners. Making sure that You cut both edges. So the angle is through the entire bit.
Very slowly saw the pieces. Only cut one edge....on two pieces. Then lay it on the floor piece of wood to check the length and mark where you need to cut the other end of the metal.
File off the sharp edges...making sure you dont scratch the top surface of the metal.
Same again for the third piece and the fourth.
This what they should look like. Longest side up, shorter side flat.
Measure and mark where you're going to put your screws. Equally on all the pieces.
Not too close to the inside corner.
Drill the hole on the central line. Drill out the hole carefully on the outside with a larger drill bit so you can sink the screws.