I have just made a composite one from the business end of a broken aluminium tiller extension (Dart 18), plastic tubing of the same diameter, and a bamboo cane slotted into both, all taped for a tight fit. It is stiff enough to push / pull against the rudders, and flexible enough to bend in a capsize and return to 'straight'. It makes me wonder why I spent £80 on a carbon spare. Mind you, I use the carbon one for real, and the "composite" one is taped to the boom as a get you home spare.