Oak works fine. Make it 68" NTN, 1-3/4" wide for half the limb and tapering to 1/2" nocks. Oak isn't the strongest in compression but is pretty toough in tension. In other words, it will follow the string a bit more than premium woods but will not break. If you can put up with a little longer bow, like 72", you can make a 50 pounder that will stay pretty straight.