The really thin 1/8th stuff seems to move around a lot (or maybe I move around a lot). It gets wadded up sometimes, and needs to be re-adjusted in my TQ.
The second option I've tried is a piece of the Thermarest Ridgecrest Solite. I cut off a part from a larger mat. This is thicker than the 1/8th, weighs little. It still isn't perfect for me though. When I carry it in my pack, I roll it up, and it stays in this shape when it's in my TQ.
The third option is an inflatable Thermarest sit pad (looks like they are called Trail Seats nowadays). This has become my go-to insulation for my feet, even though it weighs a bit more. My feet stay on top of it even when I'm rolling around.
From the other post you have on this topic, somebody threw out the idea of sewing a fabric sleeve on the outside of the sleeping bag (as an alternative to your snaps).
I did a quick test yesterday on this & like the concept, though I didn't test it exactly how it was described.
I took a loose fitting pillow case, and put that over the bottom of my TQ. I then put the pad between the TQ & pillow case. I liked how that felt. It was nice to have my feet against the TQ fabric, with the pad outside it. Not sure where I'll go from there - whether I'll explore the sleeve option, or try out the pillow case thing, but it's something to think about