But these were not sheepskin, and the "cutouts weren't really cutouts when you look close, but the panels were attached by tabs of cloth, square shaped, at about 1-2" intervals. It may have been an all purpose pad.
Grandeur saddle pads -- someone loaned me one awhile ago, and I really liked it. Cheshire Horse has them in several different styles, inluding one that accepts inserts. The material is similar to thinline but the cutouts and overall design keep the wither and spine area more unimpeded IMHO. And they're inexpensive compared to TLs. Ebay has FOUR on sale, used, at $60 apiece too.
Prolite once made a pad that was just the the shape of the panels slightly larger (basically a half pad shape) but it was two halves rather than a whole pad. They tied together front and back with small laces, so other than the tied portion the spine was absolutely completely free front to back.
I never did buy it or see it in person, but it tempted me a for a long time.
I have one that is made of kinda a thick felt. Shaped like a half-pad, two, separate pieces connected by two tabs of binding. It was advertised as a western lift pad. I use it under my treeless dressage saddle and love it. Sounds like what you are talking about.