And for your form I have a very more easy and speedy method!
Just remark that corner form is the same that the big primary form!
So just copy move it and Scale 3D !!!
And rescale it 1D for the irregular position for the "vertical" ;)
And make boolean Diff for the "Horizontal"
Seems to me very more efficient and speedy!
Scale method is very strong for simulate "box modeling" in nurbs when you have well understand the position of the "First Point"!
I have made the figure in 10 secondes ;)