A closeout fitting for a hat stringer, wherein the hat stringer comprises a pair of opposing flange portions, a pair of opposing web portions and a cap portion that extends between the web portions, each web portion extending between the flange portions and an edge of the cap portion, a closeout portion being defined proximate a gap defined in the cap portion, a channel having an interior surface being defined by the web portions and the cap portion, and an outer surface being defined by the web portions and the cap portion, said closeout fitting comprising: a cover, comprising: a fitting surface that is substantially complementary to at least a portion of the outer surface of the hat stringer, wherein said cover is configured to couple against said hat stringer outer surface; and a first portion configured to extend across the gap defined in the cap portion of the hat stringer when said cover is coupled against the closeout portion of the hat stringer; and an insert comprising a first outer perimeter surface that is substantially complementary to at least a portion of the interior surface of the channel of the hat stringer and a second outer perimeter surface that is substantially complementary to at least a portion of the fitting surface of the cover, wherein said insert is configured to couple against said channel interior surface proximate the hat stringer closeout portion, and wherein said insert is configured to couple to the cover, wherein said insert and said cover are formed with a suitable stiffness to limit torsional and/or bending deformation in the web portions of the hat stringer proximate the closeout portion when said closeout fitting is coupled to the closeout portion, wherein the insert further comprises a wall that is sealingly enclosed by said first and second outer perimeter surfaces of said insert and is configured to block flow communication through an end of the channel of the hat stringer when the insert is coupled to the closeout portion. a cover, comprising: a fitting surface that is substantially complementary to at least a portion of the outer surface of the hat stringer, wherein said cover is configured to couple against said hat stringer outer surface; and a first portion configured to extend across the gap defined in the cap portion of the hat stringer when said cover is coupled against the closeout portion of the hat stringer; and a fitting surface that is substantially complementary to at least a portion of the outer surface of the hat stringer, wherein said cover is configured to couple against said hat stringer outer surface; and a first portion configured to extend across the gap defined in the cap portion of the hat stringer when said cover is coupled against the closeout portion of the hat stringer; and an insert comprising a first outer perimeter surface that is substantially complementary to at least a portion of the interior surface of the channel of the hat stringer and a second outer perimeter surface that is substantially complementary to at least a portion of the fitting surface of the cover, wherein said insert is configured to couple against said channel interior surface proximate the hat stringer closeout portion, and wherein said insert is configured to couple to the cover, wherein said insert and said cover are formed with a suitable stiffness to limit torsional and/or bending deformation in the web portions of the hat stringer proximate the closeout portion when said closeout fitting is coupled to the closeout portion, wherein the insert further comprises a wall that is sealingly enclosed by said first and second outer perimeter surfaces of said insert and is configured to block flow communication through an end of the channel of the hat stringer when the insert is coupled to the closeout portion.