Pictures of "The Monster's" home to be.

[MitchM]

You don't need to carry the brace from the top to the bottom. Running it on the inside from the top to the side will be good enough, making a triangle.
Measure about 12" along each face and complete the triangle with a 2 x 4 cut with a 45 degree on both ends.
(this is more difficult to explain than I thought.....lol) Sorry!
Do this on each upper corner.

Mitch