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

Yah I know...so I dont inflate the thermarest completely and it remains under me...and I am not a huge person so I have more room to start with. Me thinks that the "next generation" hammocks sort out some of the problems, but I am a TW so I will wear out what I have. My biggest complaint has always been the bottom entry on the Henessey but it is workable for now; but really if the truth be known putting on the drawers, socks, and pant are easier than a side/top entry units. Had to sort out the cold shoulder blade issue and the thermarest sorted that out the first time around so I just stuck with it-and with no $'s spent. So what are the weights of these next generation no footprints? It would seem to me that they should be heavier with the additional fabric and then one enters into some mathematical arguments about that. Oh dilemmas! :-/

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.

Marlin
My BB in a compression sack and with 2 carabineers that I use to hang it weighs 3 lb 1.8 oz.  That is 8 Oz more than my HH.  Snake skins do not fit the BB very well because of the knots.   Never tried reducing the amount of air in the air mattress. Good idea.
MrB
I wrapped my over head tarp under the hammock on cold nights.  Even a single layer of nylon a few inches below the hammock makes a noticeable difference.  
MagicPaddler

Ah the creation of a dead air space...sorry for the pun  :P   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 ;)

MP - I also do not inflate my Insulmat more than 1/2 to 3/4 full, even on the ground, so it is much more comfortable than rolling around on a "full" mat.  I agree, the air is much colder than the ground, and in colder trips during Fall and Spring I use a ground tent because of that.  I think a bit thicker closed cell foam pad in the pocket under you might also help to stiffen the edges of the hammock and help keep it from enveloping you so tightly (this is only a hunch).  Regarding a quilt...I once made a small blanket out of a sheet of silnylon, a sheet of hollofill fiber, and a sheet of flannel sandwiched together like a quilt.  It was only about 36 x 60 inches, light and waterproof.  Son Dan called it the "awesome" blanket because it was so warm and usefull for being so minimal.  One last thought, whatever you use, might be made to stay put by installing snaps on it and the hammock pocket or perhaps tabs of velcro or toggles and loops???? ::)

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?

The problem with using aluminized space blankets is that they don't breathe. I tried this once on a canoe trip. I thought I would drown in my own sweat...takes wet-footing to a whole new level.

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

Another thought...kayaks come wrapped in industrial strength bubble wrap when shipped to the dealer.  Once off the kayak the piece is 8-12 feet wide and 12-18 feet long.  That's a lot of bubble wrap to play with.  You might find a dealer who will save you a piece of it.   ;)

I have a Hennessy with the "factory" insulation system but intend to replace the foam with a military poncho liner which I expect will work as good if not better and compress smaller

MP,

I bet you could use something like this for your needs?
Secure with Velcro/Snaps etc.

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This weekend, I accompanied my wife to Bed, Bath and Beyond.  (NO!  I don't usually go by myself!!!   :P)

While waiting on her to complete her shopping, I noticed a "sheet hugger" product with clips at each end.  I'm wondering if these might work for you.

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dd

ST
Your link did not work
DD
I copied down the names of the products I will go shopping.  Just my kind of store NOT!
MagicPaddler

MP, I just clicked on the link, and it loaded for me :-?

In any case, go to Brigade Quarter Masters.
Your looking for their Thinsulate Poncho liner, about 82" x 59", made out of lightweight rip-stop. Looks like you could do a lot with it?

That looks interesting.  Would make aready made under blanket.
This is the link that I got
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Thanks.  

MP,

That's the one I was talking about. I can see some multiple uses for that item with a little ingenuity?

If nothing else it would make a good emergency type blanket/ warm weather "sleeping rig" ;)

I'm sure you could use Velcro/snaps to secure in hammock somehow?