That drawing was a different take on the BB sack. I gave that one up because I felt it was to complicated to sew and wouldn't really provide that much benefit. My original idea is just like you described;basically a cloth tube, but instead of drawstrings I was going to use roll closure.
“Light thinks it travels faster than anything but it is wrong. No matter how fast light travels, it finds the darkness has always got there first, and is waiting for it."