Clear polythene. In the end we didn't use it in anger for any prolonged period so I'm not really in a position to comment on its effectiveness. On the positive side it had sat in my helmet for several years and was in perfect condition when removed from its packaging.
Lifesystems do a bright orange polythene bag. I can't comment as to whether it would fit under a helmet as I don't possess one (I have every other type going but not the polythene ones). They are easy to come by online.
Caving Supplies used to sell the small clear bags that would fit in a welly. He is shutting up shop any day now but might be able to point you in the direction of the original supplier. I'm not sure if I've seen one as small as that for sale anywhere else apart from CS.
They must be made that size for some sort of industrial use; does anyone know anywhere?
I also had problems trying to find one of these large poly survival bags a while ago. Then, after a flood, I found one floating all loose, partway through Far Sump in Peak Cavern. Gathered it up & brought it out. Hosed down inside and out & carefully dried it then folded it back up. Still using it!
It's extremely sad if we're losing Caving Supplies altogether. I've had donkeys years of excellent service there, often receiving help well beyond what could reasonably be expected. Thank you Phil and Chandra and team; enjoy your well deserved retirement!
I have some blizzard bag products. A British company based near Snowdonia. I doubt most of their products would fit in your helmet though. They do fit in an oversuit pocket or in my small tackle sack however. They are more expensive but considering you don't typically use them that often, and when you do use them you don't begrudge having spent an extra tenner on yours I can tell you that!
I don't want to distract your focus from finding a replacement, but I would consider not putting that replacement inside your helmet - the gap is not there through inefficient design! The gap is what the cradle expands into when you take a serious impact on your helmet. Leeds University materials testing many years ago showed that various items in that gap caused a significant increase in force measured by a headform sensor. I don't have that data to hand I'm afraid to support my argument - though I do remember that during some bored afternoons that a mars bar came out fairly neutral perhaps it's the right sort of squishy?
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