I bought a really soft waterproof GeF custom strap from Elliot for mine. I'll post some pics as soon as the mailman delivers the watch back from vaccum.
I also have a dark brown buffalo from Don Ginsler, but I've never had it mounted on the watch. I say black or brown leather. I'm not a fan of the gen Breitling rubber strap and if there is a gen rubber offering I think it looks strange to have it on an aftermarket rubber strap. That's just one of my quirks.
I just got my SFSO on SS and I'm thinking about one of the Rios Breitling leather straps from strappedfortime.com. The price is pretty low, so even if I don't like the strap, I won't regret the spending. Good luck.