I made one for this years use from a sleeping bag. It was just an inexpensive youth size bag i had picked up for a few bucks at a yard sale not realizing it was a youth size. I just cut it to desired size, sewed 3" grosgrain folded over around the perimeter to create a channel for shock cord, and i still need to install the shock cord and cord locks. We will ne hunting high 3rd week of sept in CO. I might freeze to death. If not i will report back. Seemed like most folks on a budget used a pad and i cant imagine a pad providing the insulation that my "quilt" will. On the down side, it is heavy and very bulky, but im young and poor.
I've debated trying my hand at making a quilt/bag but in the end I keep going back to the mantra that there are some things worth paying for an letting the pros handle. Maybe something with synthetic insulation will get me to try some day but I doubt I will ever mess with trying my own project with down.
I am in the same boat as you, looking to make a diy underquilt for my hammock once it starts to cool down. I thought about going the route of modifying a sleeping bag but not sure if I would be entirely satisfied with that.
I made a summer weight UQ from 1/2 of a poncho liner. I just cut it in half and sewed some grosgrain around it. Set it up just like a AHE Jarbidge UQ. For me it's good down to 60ish. Colder than that I take the Jarbidge.
I've never used an under quilt. I have used a regular foam sleeping pad and a sheet of that silver bubble wrap stuff you can get at a hardware. It's not as convenient as a quilt , I would guess. But it works. Of course unless I'm up in the mountains in February ,NC isn't really an issue for the most part.
If you decide to purchase rather than making one yourself the Jarbidge under quilt from Arrowhead Equipment is a pretty good value at about $100. It's a partial length under quilt but if you throw a jacket under your legs you'd be covered. Paul is great to deal with as well and frequently has sales if you know where to look.