A bit late to the party, but curious if having fire enter the chamber like that could potentially ignite some of the sawdust. If that gets clumped you're making charcoal. aditionally you would want to ensure the heat travels across the entire system. As such perhaps close the entry way as much as you can so the only exit is the other end.
It happened to me once that when the sawdust is sufficiently dry, it starts to catch fire. Of course, I reacted quickly and realized that it was not for playing. It would be more useful for drying quartz sand.
