I am working on an idea for an SPE integrated into a military Modular Sleeping System bivy. This is designed for kayak camping so weight is not a big issue. The MSS bivy is a large gore tex bivy used as the outermost layer of a multiple component system which can be tailored to meet weather conditions. I have found that the bivy with no bag on a closed cell foam mat is good down to 50 degrees when sleeping in my clothes and light fleece jacket. Adding my "15 degree" bag (which is actually good down to freezing and no more) inside the bivy and bumping the bottom insulation with a thermarest pad will get me down to the mid 20s.

In short, the idea is that since if it is cool enough to need the CCF pad for bottom insulation I always use the bivy as well, I should join the two by adding envelopes to the bottom of the bivy (its plenty big enough) to contain strips of CCF.

1) I have never used or seen an SPE but have noted that the original Speer design used spandex on the back. Other diy designs used some sort of elastic, but I have never understood why. Why not just have an envelope of all the same material, why spandex?
2) How do backpackers carry an SPE? Do you roll the whole thing up an carry it vertically on the side of the pack or strip it apart and carry the components separately?