are you mitering the corner? the depth is usually achieved by contrast in the fabrics- and wider sashing may help too-
i would fold some fabrics at different widths & lay them around the block to see what looks best-
and attic window blocks are not always square- they can be any rectangular size/shape. check out the book- Attic Windows (your library may be able to get it for you- you will see some ideas that help with contrast & depth.