A method for forming a composite structure, the method comprising: using a die assembly to cooperate with a mandrel assembly to shape a composite sheet into a shape suitable for insertion into a bottom end of a composite body, wherein the mandrel assembly includes an outer mandrel and an inner mandrel, the inner mandrel comprising a first mandrel surface adjacent to a second mandrel surface, the first mandrel surface and the second mandrel surface being aligned to one another at a forming angle (Φ); positioning the composite sheet adjacent to a die opening of the die assembly, wherein the composite sheet has a first sheet surface and a second sheet surface that define a sheet thickness of the composite sheet there between, and the composite sheet comprises a fiber layer, an oxygen barrier layer, and a sealant layer; constraining a portion of the composite sheet between a first forming surface of the outer mandrel and a second forming surface of the die assembly, wherein the first forming surface is spaced a gap distance from the second forming surface, and the gap distance is substantially equal to or greater than the sheet thickness; urging the composite sheet through the die opening and along a third forming surface of the die assembly to form a composite bottom from the composite sheet; inserting the composite bottom into the bottom end of the composite body such that the composite bottom is at least partially surrounded by the bottom end of the composite body; sealing the composite bottom to the composite body; wherein a sealing member is configured to provide heat and pressure for heat sealing, the sealing member being positionable between a sealing position and an open position in that the sealing member is rotatably coupled to the die assembly; wherein the bottom end of the composite body is contacted with the sealing member on the exterior surface of the composite body such that the composite body and the composite bottom are compressed between the second mandrel surface and the sealing member; wherein the composite bottom is hermetically sealed to the composite body; wherein the sealing member is moved away from the bottom end of the composite body. using a die assembly to cooperate with a mandrel assembly to shape a composite sheet into a shape suitable for insertion into a bottom end of a composite body, wherein the mandrel assembly includes an outer mandrel and an inner mandrel, the inner mandrel comprising a first mandrel surface adjacent to a second mandrel surface, the first mandrel surface and the second mandrel surface being aligned to one another at a forming angle (Φ); positioning the composite sheet adjacent to a die opening of the die assembly, wherein the composite sheet has a first sheet surface and a second sheet surface that define a sheet thickness of the composite sheet there between, and the composite sheet comprises a fiber layer, an oxygen barrier layer, and a sealant layer; constraining a portion of the composite sheet between a first forming surface of the outer mandrel and a second forming surface of the die assembly, wherein the first forming surface is spaced a gap distance from the second forming surface, and the gap distance is substantially equal to or greater than the sheet thickness; urging the composite sheet through the die opening and along a third forming surface of the die assembly to form a composite bottom from the composite sheet; inserting the composite bottom into the bottom end of the composite body such that the composite bottom is at least partially surrounded by the bottom end of the composite body; sealing the composite bottom to the composite body; wherein a sealing member is configured to provide heat and pressure for heat sealing, the sealing member being positionable between a sealing position and an open position in that the sealing member is rotatably coupled to the die assembly; wherein the bottom end of the composite body is contacted with the sealing member on the exterior surface of the composite body such that the composite body and the composite bottom are compressed between the second mandrel surface and the sealing member; wherein the composite bottom is hermetically sealed to the composite body; wherein the sealing member is moved away from the bottom end of the composite body.