I've been tossing the idea around for a bit and finally re-setup my indoor stand so I could try some things out and I think I've got it working good for me. Gave it my normal fan on high test to see how well it was insulating and it was working great. The heat between the hammock and bag was amazing when I stuck my hand between them to feel.
I passed some 550 paracord through the ends of my suspension and tied them together making a large loop from end to end on the hammock. I first tried this with shock cord, but it was too light to hold the bag up good, its a pretty hefty down bag. The 550 stretches just enough but keeps the bag up high on the sides of the hammock when in it.
I then took the opened bag and ran it under the hammock and the sides up and over the paracord draping it over. Knowing the bag would slip and end up on the ground I took a pair of tarp clips with shorth length of shock cord between them, then hooked that in the middle. Clamping on the zipper of each side. I made 2 more of these with shorter cord and put them on the foot and head end. A loop of shock cord from each of the head and foot to the suspension with a mini biner to keep the bag centered in the hammock.
Tested it out and it worked out great. The best part is no modification to the bag and easy to setup and take down.
The whole bag shot...
Close up of the center clip
The foot end... The ribbon looking stuff is the ties on the end of the bag. I used one to tie the zipper to the clips. The head has a draw string and its pulled about a third to close up the end.
The bag conforms to the bottom of the hammock really well, it rides against the hammock from head to foot, but not tight enough to compress the down.
I will probably replace the 550 with some stronger shock cord, but as it stands now it works really well. The sides of the bag are high up to the top of the hammock, no cold shoulder at the top of the hammock.
I'm sure its not an original idea, but it worked good for me until I can hack the bag up for a purpose built UQ.