Well I don't want to lose conditioned air through the walls or ceiling. This hab has a 1/8 inch thick wooden lid. I'm sure enough air gets out around it.
Since I want to spot-cool, it looks like I'll have to just build it before I know whether the room air will sink down the tube. I guess I'll have to use smoke at the top to see if it flows down or just drifts near the tube's entrance. My herp will have to be evicted for that test.