I made an insulated cuben hammock, then turned it into a cuben underquilt. It works, but I need to pay a lot more attention to controlling my body temp, and I'm usually aware of a slight tendency for condensation to form (on the good
days). I think cuben is better for tarps, dry bags, packs, and some rain gear. It's not a good choice for quilts or hammocks. I would still use the lightest cuben (1/3 oz/yd) for baffles in a quilt if I wanted it to be ultralight, but I'd try M50 for the top and bottom layers. For a still very light quilt (but not ultra
) I'd use Pertex Quantum, Pertex Microlight, or Momentum 90. That would save a lot of money, but might add an ounce or two. (There are other new lightweight fabrics that I'm not familiar with.)
One other thing about cuben quilts - they need a breathable fabric panel to let the air in and out. The location and size of the panel affect how easy it is to compress the quilt for packing. If you trap air in the cuben portion, compressing the quilt risks popping it.
Humid environments add to the challenge. Some might recommend synthetic insulation, but I think down can be managed.