I have been using handwarmers in my setup for a few years and I want to share an observation.
Putting your handwarmers under your hammock, in your underquilt is a much more efficient use of their heat. The handwarmer, instead of heating you, radiates heat into the down, keeping you warmer than if it were in the hammock with you. The toe warmers have adhesive so you can stick them where you like. I have thought about sewing some small pockets on the inside of my underquilt for holding the handwarmers
That said, I am a cold sleeper, and if it's going to be very cold, I will have plenty of these above and below in the hammock with me.
Another way to get more heat out of them if you keep them on you is to put them under your armpits and in your groin.
The rounded toe warmers with the adhesive also fit nicely onto the armpit of a tshirt that you can wear while sleeping or throughout the day.
Just thought I'd share