Evon,
I would recommend using a metal axle on a diablo. If you make it out of wood, it will be weak and a certain breaking point when the diablo is dropped. Also, I don't think that the wood axle would allow the diablo to spin for very long on the string.
I've only used diablos made from rubber with metal axles, so I can't speak from experience with wooden diablos. However, with as much abuse as my diablos have taken over the years, I doubt that a wooden diablo would have lasted very long.
One other thing to consider is balance. It may be fairly tough to get a wooden diablo to spin true.
Good luck, and let us know how it works out.
Matt