I use an REI lite core - it keeps my feet toasty and it feels great to prop my dogs up on it after a long day walking. Also, it packs down right small.
i see where you're coming from hobie. i have to keep mine in my topquilt b/c i shift positions throughout the night. that and i don't always use a double layer hammock. to me the OP was just asking for a good way to go about it. i chose the simpliest, lightest, and least expensive option. you're right though that there are dozens of configurations...
It's rare that I need to use one since my legs hardly ever get cold. When I do though I use the foam insert back pad that came with my ULA Ohm. For me it is the perfect length and width.
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I've been stuffing my spare clothes in either my pack lid or a spare stuff sack inside my bag as a pillow for my feet and it worked great even in 20 degree weather (using a AHE Flamethrower under me)
I use a cut down piece of Thermarest ridge rest closed cell pad. It doubles as the back pad in my REI flash60 hat I gutted to make it framless. It is very small maybe 14x14 in.