I am a member of the Giraffe-Leg-Club as well. I am 5’10 and 145 lbs. I also have a short-backed horse who also happens to be a true narrow with long, high withers. I feel your pain.
I work in a tack shop and I sit in saddles when the days is slow. It is different than riding in them of course, but my knee sticks off the front of 90% of the saddles that we have so I wouldn’t bother to ride in most of them anyway.
I would stay away from Stubben close contact saddles for the most part, although I do have a Stubben dressage saddle that I think is pretty perfect for me, and fits nicely on my short-backed horse.
One of the only ones that my leg fits on is a 17.5" County Extreme. I like the leather on it too, and even though the seat was probably too big for me, I didn’t feel like I was swimming. County in general has a lot of flap options, so maybe you could contact a local rep, even if you are looking for used. They should have access to a network of people who could help you locate something that will work for you. The one thing I will say is that the panels on the County Extreme are quite long lengthwise and I doubt it would work for a short-backed horse.
A 17.5 M. Toulouse monoflap fit me, although the leather was horrid and slippery. My butt was also much too small for the seat.
And finally, the craziest one that fit me, and fit me the best, was a 16.5" Crosby Hunterdon. I’d bet money that it is the same saddle that is referenced in post 4. I bought it too, and I am happy with it. It is so tiny that it swims in any saddle pad that I put under it, so I think it would be fine on a short-backed horse. The stirrup bars may be a tad forward for most long-legged people, but it is such a minimalist saddle that I don’t feel like my balance is compromised. And because it is old, you could probably find someone to add a knee block if you really wanted. Finding tack to match it even remotely is a whole other issue though, and if anyone has any suggestions about that, let me know 