Step three: Now the ukulele makers are ready to build the sound box. The side pieces must be very straight and must be 3/32 of an inch thick. These side pieces are sanded and dampened so that they can bend. Then the koa wood strips are clamped to a metal mold and heated with a propane heater. This takes out the moisture without burning. The pieces then are pefectly shaped and ready to fit the next mold.