Just use a pipe with conductive plating on the inside and operate it in the 3-digit MHz range as a waveguide. Make sure that the core of the wood has more power than the outside, and that you e.g. move it around lengthwise if you get longitudal resonance in the pipe. Add some desiccant, and make sure the way you couple the energy into the chamber can handle this resonance without frying itself through reflection. Might not need the vacuum, but it should be easy to construct it in a way that it can hold a vacuum of around 10~200 mBar, e.g. with a simple one-stage pump. If you make sure the pump doesn't take in much moisture, there shouldn't be an issue for it.