I just played with it some more. There was a flake of metal stuck to the end of the horn, which I simply removed with a fingernail. I also gently buffed the end to a shine with a polishing compound just to make sure it would make full contact with the diaphragm. It appears from slight wear marks that the horn is fully contacting the washer that's bolted to the diaphragm when it's screwed down until it just touches. I have also run back through the contact adjustments each time I change something else.
Also, I've been doing tests with the case open, so I can make adjustments. Does the case need to be sealed up for it to work? And, should there be some kind of gasket at the threaded end of the projector, or should it contact the diaphragm directly?
After all this, still no progress. I'm about ready to give up on it, and in a few days I'll to be moving to a place where I can't constantly test it while I'm trying things.