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.
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.