After leak testing the fitting I built, I maneuvered it into place in the overflow box, only to find that the threads on the inside of the bulkhead are larger than 1" (which surprised me since 1" PVC fits into the slip fitting on the other side). Anyway, this turned out to be beneficial, because I think screwing this fitting into the threads in such a small space would have been tough. With the intake on my new fitting pointed downward, I fired up the pump. Of course, the fitting sucked a lot of air, though it was a small improvement over the naked bulkhead.
I had planned to unscrew the fpt x hose barb elbow that is currently attached to the outside of the drain bulkhead in order to attach a 1" PVC tee for the air intake on the durso, but I decided to just drill a hole in my new fitting and attach airline tubing for the air intake as a quick test. The water level in the overflow box kept see-sawing, so I removed the fitting and drilled a hole to install a second piece of airline tubing. When the water level continued to see-saw, I went back to the durso website
http://www.dursostandpipes.com/ and discovered that an oscillating water level in the overflow box is usually due to excess back pressure in the drain (caused by the drain outlet being too far submerged in the sump) rather than by an incorrectly sized air intake on the durso. I raised the drain hose to the surface of my "sump" and reinstalled the fitting. This time, the water level held steady, and the loud slurping sound was replaced by a noise like someone sucking the last of their drink through a drinking straw.
The water splashing into the overflow has an audible trickle to it, but this was diminished somewhat with filter floss. I estimate that I reduced the overall volume by about 80%, and the grating, annoying quality of the noise by about 90%. It's not completely quiet, but at least I can watch TV without turning off the return pump.