Here is what I used to do before I got an underquilt. I laid a sheet of JRB foam in my hammock so that it would wrap around my torso. Then I put my thermarest on top of that and slept on the thermarest (the two pads formed what looked like a cross in my hammock). I did this because i liked the idea of a SPE but was too lazy to make one. The JRB foam is tacky enough that it didn't slide around under my thermarest, and the thermarest prevented it from getting all bunched up and uncomfortable underneath me when i rolled over. It worked quite well and I got into the 20s with this system without any problems at all. Plus I already had the thermarest and the JRB pad was only ~$10... pretty cheap solution.
My point is, I took a lot of the info i read about and mashed them up into a single system that ended up working pretty well. Don't know if that is helpful, but it's my two cents.
I went to the woods because I wished to live deliberately, to front only the essential facts of life, and see if I could not learn what it had to teach, and not, when I came to die, discover that I have not lived.
-Henry David Thoreau