I made a trial underquilt for HH ultralight asym last year. My first try worked OK, not perfect to be sure. It uses round elastic about 1/8," I think a little thicker, what they had locally, maybe too heavy. Put the elastic in edge sleeves, just the folded over outer shell of the cheap nylon coat liner material from JoAnnes Fabrics. It makes a circle, emerging from the edge sleeves at each end. I stretch the cord out at both ends and use the hooks that HH has for the stock rain fly, slide them up the cord to tension it more.
Need to use a glove hook at each cord knot on hammock ends to keep the elastic high and the quilt up against the bottom. It's hard to get it tight enough but not too tight when you get in. This is a starting point for experimentation IMO. It's helpful to start with this I think because it allows a lot of adustment. If you want to use another attachment idea, just pull the elastic out. Add an inner liner if you want once you get it working. For tries just leave the insulation bare inside.
I used thinsulate too, one layer of the thinner material, nominally 1/2" I think, but it lofts thicker seemingly. This and the outer shell is cut into a canoe shape, exactly the term I used for it. I made a cloth pattern from a cheap sheet first, fitted by draping over the elastic cord loop and pinning it. Just the two end seams, the curves at the head and foot of the canoe. There is some gentle curve in the sides. Thinsulate cut this way too, end seams hand sewn in big whip stitches with narrow strips of cloth along the sew line, laced up really. The edges of the thinsulate canoe are trapped and sewn through the outer nylon shell, cut to the same shape but a little looser, which I sprayed with the standard outdoor store DWR spray.
This works OK for a first try. I am about to attach this quilt without edge elastic to a homemade open quilt a la Risk by just sewing it along the edges and seeing how this works out. I find the bottom quilt the best answer. All tries at insulation inside the hammock were a frustrating experience I don't want to repeat. I'd love to have a bottom quilt permanently attached and always in adjustment. We'll see!
Best, Todd in Tarzana.
P.S. I went back and doubled up the insulation in the head and torso area, and down below the legs using leftover thinsulate. Stuck it to the original layer with 3M adhesive spray. This works, but I'd suggest using the thicker thinsulate in the first place. But if you want to add some the 3M works OK.