I posted a week or so ago about just taping pads together to simulate a SPE. The whole reason I started thinking about this was to be able to carry a 30" pad in or on my pack. The more I thought about it, the more I realized that the pad would still be too thick when rolled up.
Here's my solution. From Warbonnetguy's recommendation I ordered some landau foam from: http://www.a1foamandfabrics.com/site/index.cfm. I used a 3/4 length of 1/4" foam cut 24" wide as my main insulation. Under this I used a 3/4 length of 1/8" foam cut 30" wide. The whole setup weighs 10oz.
This gave me 3/8" inch of foam underneath my torso and 1/8" for my shoulders and knees. This setup was somewhat easy to get in my pack. I used the 1/4" foam rolled around the inside of my pack with everything stuffed inside it. The 1/8" was rolled up and then folded in half and placed in the very top of my pack. I think that this setup is much easier to handle in a pack than one piece of 3/8" of foam and provides nearly as much warmth.
I tried this setup out this weekend with the low around 50 degrees. I was toasty hot and I'm a skinny cold sleeper. My last time in the hammock was about the same conditions and I used an old red thermarest. The landau pad setup was much warmer and I didn't get any cold spots on my shoulders or knees.