Drew at RevHiker repurposed an old sleeping bag for his DIY hammock underquilt with the help of a pocket knife and some duct tape.
A regular sleeping bag doesn't do a whole lot to keep you warm in a hammock, because it compresses underneath you. Without the fluff, you get no insulation from the cold.
You can solve that in one of two ways.
You can use an insulating sleeping pad in your hammock.
Or you can use a specialized sleeping bag called an underquilt. It's a sleeping bag that goes around and under your entire hammock so that it doesn't get compressed and so that you stay warm.
The problem is, hammock underquilts can be expensive. That's why Drew made his own.
He cut holes at both ends of the sleeping bag to string the hammock through it. He patched up the edges of the holes with duct tape. Simple.