I had trouble doing this once before, but tried stacking my 20 and 40 Incubators again. 20 on hammock, 40 on the outside. This worked really well since the 40 has the old suspension and can be set up MUCH looser. I set the 20 up as usual, then lightly hugged it with the 40. Stays in place without any issues.
I got in and realized I have a BLAST FURNACE! I bet this would get me to 0 or lower. Not the bulk I would want to backpack with, but for a really cold winter trip here and there it may get me by to save some $$$.