There is nothing to hold it in place, but it might work just fine.
You can order a Speer Pad Extender and the CCF foam from Speer Hammocks.
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The pad would have to be flexible enough to let the SS form to the hammock. Since the pad is cheapest part of the system I'm not sure why you would want to replace it with something else.
The Torso and Kidney pads are available from HH. Or at least were. I don't understand why they don't advertise them when there are reveiws and quotations from Tom H saying they are an important part of the system. Ask for them specifical;ly when/if you order the set up.
Yea I haven't even seen them mentioned on the Hennessy website. Are they additional pads to add to the UnderPad or is it a modified UnderPad with the kidney and torso pads added?
They are additional. You can add the torso size pad, the smaller kidney area pad, or both together to the original full length pad. It doubles or triples the thickness at the area where you need it most, for a pretty small weight/bulk increase. I have found them quite helpful.
Or you can skip all of that and use clothing you are not wearing to bed, or a Garlington Insulator ( several threads here on that) for a significant warmth boost. Or you can use all of the above with the kidney/torso pads. There are quite a few tricks for boosting warmth in the SS.
Of course, if you are going to use extra clothes in the under cover or to make a Garlington Insulator, it will require that you have some extra clothing that is not already a part of your top quilt/bag sleep system. If you are depending on every piece of clothing you have being worn while you sleep in order to be warm enough on top, then naturally you can't count on adding it below for extra insulation.
Should my UC (Sil) be hanging free and clear under the pad and hammock creating an air gap? Last night was my first hang and it seemed that the UC was tight to the bottom of the pad, and both were covered/soaked in condensation by morning.Soaked is an overstatement, but the pad is still moist after hanging in the garage since lunchtime.
Maybe my Heat sheet was not covering me completely, but apart from issues of placement at the feet, it certainly felt like there was coverage.