Good idea Arizona, the Neoprene probably would be lighter than a double wall. But, as far as insulation goes, neoprene is pretty heavy. I wonder if it would be better to wrap it in Styrofoam?

I have a thin plastic to-go container, the kind you get soup in from a restaurant, that I put inside a Dairy Queen drink cup and then cut off the excess Styrofoam hanging off the bottom. I haven't ever actually used it though; it's just something I was messing around with. If I have the time, I'll put some hot water in it and see how long it stays hot. One nice thing about it is the to-go container has a lid. I need to glue foam to the top and bottom of it as well. But, even if it works, that wouldn't be appropriate for to carry hot liquid inside your pack with. It would be too easy for the lid to come off or for it to leak. Might be just the thing for an extremely lightweight food bowl and tea cup. It might also give some insight into how well Styrofoam could work on a bottle with a better lid.