What type of hammock are you using? I think it is possible, but with a good down bag there maybe an easier way.
All my DIY underquilts are is a big rectangle with drawstrings on the ends to fir it to the hammock. You might be able to cut the bag to a rectangle, sew, add drawstrings, and add tie-outs and be done.
Do some measuring an try holding it to the bottom and see how it looks. Also see if the ends would work well with a drawsting.
Is that too much to ask? Girls with frikkin' lasers on their heads?
The hanger formly known as "hammock engineer".