At this point all 4 edges are open; let us close them.
Now turn the paper over and valley-fold about 1" of the top and bottom edges.
For the right edge, valley-fold at an angle so that bottom part (2") is farther inside than the top part (1"). Repeat the same on the left edge.
Crease firmly. While doing this the side edges will tend to go out, which is OK. This will give an extra bit of room in the pocket.
You'll find the top corners falling outside the top edge line. Bring the corners inside by tucking them in.
The letter holder is now ready and should look like an inverted trapezium.
Dimensions of the finished model are - Top length: 21"; Bottom length: 19"; Height: 13". [Note: folding the sides at an angle would give it some interesting shape rather than keeping it just a rectangle.]