I was heartbroken that the Spirit Flyback Titanium only comes in 42mm, so I did a thing...
I really, really wanted the Flyback in titanium, and even though the lugs didn't hang over my wrist when I tried it on (I have a very flat but pretty narrow wrist), it was more of a "statement watch" than I was going for.
The 39mm flyback in steel is HEAVY, so that was out (for me personally), and I loved the matte anthracite dial on the 42mm Ti anyway, so I ultimately went with the 39mm Spirit Zulu Ti. I just couldn't shake the image I originally fell in love with, though, which was how that color scheme worked on the NATO.
Turns out the 42mm flyback NATO fits the 39mm Zulu! I was skeptical, since the listed specs on the lugs are 21mm against the NATO's width of 21.5mm, but Longines seems to be about as good at math as I am, and it worked out. For some reason, though, they're not selling the buckle with it, so I had to convince an authorized dealer to order it for me as a repair part.
So yes, I totally just overpaid for a scrap of nylon and spent a ton of time I didn't have to score a buckle, but... The things we do for our love of watches, I guess?
(edited to clarify that this watch is the 39mm Zulu Titanium)