If you use a pad inside, it needs to be 24" or wider. I found that a slightly tighter hang worked better. You may get some buckling. Rounding the corners helped too. The Gossamer Gear wide Thinlight pad is a good way to go. Internal CCF pads are by far the least expensive and lightest option for hammock insulation, but not the most comfortable. That stiff layer of foam really detracts from the comfort of the fabric surface.
Hanging a regular CCF pad under the hammock will leave all kinds of air gaps. You will get more wind cutting action than actual insulation value--- in other words, it won't do much for you. A space blanket hung tight under the hammock would do as much or more. The Hennessy works because it is soft and flexible and has the undercover to add another layer for dead air space. Note the Hennessy is open cell foam vs the standard sleeping pads that are closed cell foam (and won't absorb water).
You can buy a ready-made undercover from 2QZQ and use Garlington Insulators inside (http://www.garlington.biz/Ray/Hammoc.../Insulator.htm
). I fiddled around with all kinds of combinations and ended up buying the Hennessy foam pad by itself and using my own undercover. Adding a space blanket does help with the Hennessy system. IMHO, the SuperShelter is summer stuff for above freezing, but it is comfortable. You can stack it with other options like an underquilt or the Garlington Insulators. Many have used the SuperShelter to lower temps, so YMMV.
The Wilderness Logics 3/4 synthetic UQ will work fine with the Hennessy ($115). I think it is the least expensive yet effective insulation option for 3-season use.