The harness bits (and to a modified degree, the belt ends) remind me of the latches on the five-point harnesses in Grandpa's Citabras and the Great Lake (aerobatic planes). I was supposed to have been out the door half an hour ago but I can't resist hunting around a bit to see if I can find anything useful.

OO! First pic on this page! I searched a bit more on 'rotary buckle' with limited results but I'm rushing now because I really do need to scoot. But I came across the Hooker Harness site by searching for homebuilt, aviation, and harness (or buckle, I forget now) and there were some other possible vendors. Maybe that will at least give you some keywords to hit up eBay or Craigslist or something for used bits on the cheap?

Good luck, can't wait to see how it comes out!