Lots of good ideas!
Marlin
I would miss the wonders of a hammock.
IC
I think a under quilt of that size would cover the spot.   I have thought about adding clips like clothes  pins to the corners and clipping it on.  To work best it needs to fit close near the edges and loose in the middle.
ST and OS
If  the space blanket were used like IC under blanket it probably would work well.  Humidity goes up and if this is under you I don’t think there would be the condensate problem that you get with it over top of you.  One time when I wrapped the top tarp under the hammock I wrapped it up a little to tight and got that clammy wet feeling.  The space blanket would be light.

I found some 1/8 inch packing foam. When the weather warms up a bit I am going to try it in between the two bottoms.
MagicPaddler