I tried the girth in about four different places on here, and every time it slid all around. She's just really thin, there's hardly any contour to her chest at all. I had it as tight as I dared go today, and it still went everywhere.
I'll look into that wintec, I have another one and I love it so much. It was fitted to my slope backed gelding (I have a horse in every shape and size). I couldn't find the Duett saddle you were talking about, though.
Thank you all for your answers!