No, I'm not. Consciousness is a continuum, not a dichotomy. There are nonetheless systems whose degree of consciousness is indistinguishable from zero.
> there's no hard evidence about how low the scale goes.
Of course there is. It goes down to zero. Rocks are not conscious. Dead people are not conscious. Even humans under general anesthesia are not conscious.
Yes.
> If so, how can you be certain?
I can't be absolutely certain of that nor of anything else. The philosophical-zombie hypothesis cannot be ruled out by evidence. But I observe others producing complex I/O behavior that is qualitatively similar to that which I produce and that I attribute to my own consciousness. I also see evidence that my consciousness supervenes on the operation of my brain, and I observe that other people have brains. The most parsimonious explanation for all that is that other humans are also conscious.
Nope.
> Can we construct one from legos in an aircraft hangar?
Almost certainly not. We can't even construct one on a silicon chip (yet).