Jimbo
I have tried a pad in the Hennessey and so has Quentin.  Quentin usually wakes up in the middle of the pad only under it.  I tried using a pad a couple of times and found it hard to get in the hammock in the sleeping bag and on top of the pad.  You can never have too many canoes or hammocks so I bought a new hammock.  My new one is a BlackBird Warbonnet.  It is a asymmetric so you lay flat and have shoulder room so I can sleep on my side.  It is top entry making it easer to get in and out.  It has a double bottom so your foam pad does not slip around.
All right if inney verses outey is old how about asymmetric verses peapod!

Db
The air definitely sucks more heat then the ground. It can be like sleeping on a cold waterbed with a electric blanket on you. Your body is cold so it produces more heat but the heat does not go to the correct spots.  I can get 1/8 and ¼ foam pads. I would rather deal with the 1/8 if it will do the job.

Marlin and ic
I have used a air mattress in a big Agnes in my Hennessey.  It kept me warm except it was not wide enough where the hammock comes up on the side and rests on your shoulder.  It was too thick and made the hammock not lay as comfortable. If I were on the ground I think the inflatable would be the best.
MagicPaddler

The answer is not a pad - its a quilt slung under the hammock. There are various designs people have come up with.  Mine has a very light thermal blanket inside it too.

Ah the creation of a dead air space...sorry for the pun     the "quilt" is a very good idea. So 3# mark is being touched then why not a solo tent? More space for the necessities, vestibule, easier changes...and yes I understand the wonders of a hammock, to be sure. Just a thought

Just thinking out-side the box.
What if you took 2 aluminized space blankets, and lapped them over, end to end and stitched together. You'd want it long enough not to crowd overall length of hammock. Then stitch up each short end leaving a open slot on each end enough to get end ropes thru. Then finish stitching the last 4" or so on each side.
 You'll have an "envelope" open at the top and enclosing the hammock.
It would be a wind/rain break, reflect heat back, even a spot for storage.
Wouldn't add much weight and you could probably stitch on home sewing machine? You'd have to play with the dimensions of hammock and slack needed, but it might give you the extra warmth you want?

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