I settled on what is essentially Bubble Wrap surrounded on either side by Aluminum Foil: http://geni.us/EcoFoil
This stuff works primarily by reflecting any radiant heat that comes in contact with the aluminum. My makeshift propane stove heats up several firebricks which radiate heat into the yurt, and that heat is bounced around inside the yurt rather than leaking through the porous cotton fabric which was the only thing there before.
It was really easy to install along the walls by sliding pieces I cut between the cotton cloth exterior and the wooden supports inside. It feels like the Yurt was designed to make this as simple as it could possible be! I thought the ceiling was going to be a challenge due to the non-rectangular angles, but discovered that it worked perfectly if I cut a diagonal line down each rectangle of insulation.
So far it’s working out beautifully! I am easily able to get temperatures 30-40 degrees Fahrenheit higher than the temperature outside. I’ve been able to keep it in the 70s in here when the temperature outside is nearing freezing!
I’m really looking forward to the challenge of temperatures dropping another 20 degrees in the Winter. I’ll provide updates on how well the insulation is working once it’s starts getting really chilly.