My preference for fighter kite spine material is bamboo. The variety of choices bamboo has to offer is up to now unparalleled. The bamboo spine is trimmed to shape, size, weight and stiffness/flexibility from a larger piece. Each spine is custom tailored to fulfill particular design parameters.
If you have an aversion to bamboo or would like to try something new, here are two alternatives worth exploring that provide good results.
The first one was initially shown to me by Johnny Hsiung. The spine is made from a fiber glass or carbon fiber rod running through a section of aluminum tubbing. These two materials contain one another. The rod prevents the tube from kinking and the tube allows the rod to hold the desired curve. It is first necessary to know the dimensions of the kite and the placement of the lower bridle point. The example I saw ( and copied ) had the tube run from approximately one inch above the bow to just short of the lower bridle point. Take care when handling the tubbing for it is very susceptible to misshaping. To cut, slide a length of synthetic rod inside the tube insuring support where cutting. Rotate the tube while gently cutting with scissors. Just a little at a time. Sand all burs off of cut ends with 400 grit abrasive paper. Deburr the inside diameter of the tube by rotating over a crochet hook or like object. Make sure a length of rod will easily slide through the tube before construction. Glue or tape the tube to the sail material. The rod can be cut too long for ease of handling. Run the rod through the tube and secure at the nose of the kite. Trim. Secure the rod at the tail. Trim.
This spine can be shaped to suit when the kite is ready to fly. In the event of a crash ( nosing in ) it will be necessary to reshape the spine. The aluminum tubbing is available at finer hardware stores. The inside diameter is approx. 0.065. The fiber glass rod is 0.063 and the carbon fiber is 0.060.
This second spine was first shown to me by Ed Alden. The spine is made from carbon fiber secured from nose to tail of the kite. The bend is made possible by clever placement of cleats along the length of the rod. The cleats allow an adjustable string to be tied on in a manner that induces bending of the rod to the desired spine shape. Elastic bands may also be employed in place of the string. The bands allow a constant flexing and recoiling of the spine shape during flight.
Carbon fiber rod when bent has a propensity to settle. Hold a bent length with the hands and rotate the rod. You can feel it pop into a predetermined position. Before securing the rod to the sail material or installing permanent cleats, insure the rod is rotated in the correct manner.