FWIW:
I chose cap screws at 1/8" BSPT in stainless (McMaster 4810T191, $2.63 each). Thought about hex-head plugs, but decided on cap screws (which are essentially just bolts that have thread on the full shaft) because I thought they might seal better and be less likely to move over time. And either choice is about equally unobtrusive.

I had picked up some SS 1/8" NPT also, but didn't use them because I didn't like the idea of re-tapping or otherwise damaging the original threads, and also, however unlikely, I or a later owner might want to replace the air injection some day.

Andrew.