I don't even have the hammock yet, so I'm a way of from starting this. I don't have much left over for an underquilt, so I wanted to make my own.
I have an old polyester fill sleeping bag that already has a tear in it I could use to make the underquilt. My only concern is how cold polyester may be good to. beginning of summer, I'm not too worried yet, though I should likely start making plans to make another once it starts getting warmer.