Mine aren’t inserts - I just have two pads (and have NEVER had any issues with shifting or slipping).
I have THIS saddle pad.
And set THIS ThinLine pad on top.
Of course - my set up was a bit more economical because I am a used tack sale junky. Picked up the CoolBack pad for $15 at a charity tack sale, and the Ultra ThinLine for $1 at the Stanford U. tack sale! (still says “Nina - Stanford” on it hahah)