I feel your pain. On several occasions I have had to make an allen or square wrench due to corrosion, over torque by ham fisted operator, or just loose fit. Sometimes you can carefully reduce the next size by careful grinding (not overheat). I have also made them with hardenable steel stock in soft condition. When fit is perfect heat dull to cherry red and hold for couple minutes , then immerse in a little can of engine oil. Finsh grind if bigger after heat treat. Just did this on a Koni shock shaft tip.