I converted a synthetic sleeping bag with Primaloft insulation (only about .75 inch loft for 40 degrees) into an under quilt. I tried the old method of putting on four loops and a cinch tunnel with cord, but it would not come up tight to the hammock uniformly.
So, I sewed some hook velcro to match the Speer type velcro along the edges of the the DIY hammock, sewed on many loops on each end and sewed them only to the inside piece of fabric that will be touching the hammock. Then I cut individual bungees with loops at both ends so that just a small amount of pull would be on each one. I think there is eight on the foot side and ten on the head side.
Lastly, since it still didn't pull up as close as I liked, I put my sewing machine on the longest stitch I could and sewed pleats so that the inside piece of fabric would be shorter than the outside piece of fabric. (please excuse the sloppy sewing in the picture, this is just a test at this point) This is similar though simpler than the Speer Snugfit design, but it made it so when the inner layer is pulled up against the hammock using many bungees, distributing the force evenly instead of just a cinch cord and two bungees per end, the hammock came up tight to me and the hammock but stayed loose on the outside layer, leaving the loft intact completely. I slept in the hammock last night at 48 degrees and it kept me warm all over. Here are some pictures.