On our most recent camping trip my mate simply used his frost Nalgene bottle along the same lines, with the headlamp sitting beside the Nalgene not in it . It worked surprisingly well.
Clever , simple design. Maybe at 75g the Luci Emergency light (solar powered) could be better after a few days of use. The difference in weight is probably similar to the extra batteries needed for the headlight and you have a spare light in case something happens to one of them.
It is a clever option, simply a bag. Sierra Designs make some good gear. If, to buy something (rather than myog), the petzl e-lites are great! You do need somewhere high to hang but something like 45hrs on a low, diffuse setting (not a lot less on high) and 25g's, plus the fact that you can have red , they're a classic
kitty posted something similar, but much lighter in my thread on a DIY lantern diffuser - the Montbell Crushable Lamp Shade (or Crushable Lantern Shade in the US) that weighs around 5g. viewtopic.php?f=15&t=20636
I use 1xAA torches rather than headlamps so my DIY lantern diffuser using a Berocca style tube comes in at about 15g (less if you don't take the lid), but I use it as storage for my matches and flint when not using it as a lantern.