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???? ::)