On my frontcountry experience, for bottom insulation, I used Gossamer Gear's 1/4" Evazote Foam pad. My DD hammock is double layer so the pad inserted between the layers and at 39" wide was easy to stay on top of. For top insulation I used a GoLite RS 1+ quilt, which has been my "summer bag" for several seasons. Combined, the pad and quilt weigh in at 2 lbs, 5 ounces. This certainly isn't ultralight by today's standards, but my first thermarest alone weighed more. It actually made it down to 41 the 3rd night and I stayed quite warm.
For the October trip, depending on forecast I might bring a MH Ultralamina 32 bag instead of the quilt for a bit more warmth at the expense of 6 ounces.
If after several seasons I still really like the hammock camping thing, I will probably switch to an underquilt. UQs are often sized to the specific hammock though, and the double layer hammock body is no longer is necessary to hold the insulation in place. I will probably replace my hammock before getting an underquilt for that reason..
Underquilts are certainly the way to go. I don't hammock camp, but my brother does. He has a hammock gear underquilt that is great. My understanding is that they are not sized to specific hammocks, but they are made in different sizes. If you don't have full coverage, or choose a 3/4 length, then you can just cut the GG sleeplite down and place it under your feet. If your pack is small, you can use that for a pillow. Things like that help cut weight down and make gear more versatile.