In reply to Si Withington:
My Ion is fairly roomy but I still find with the mat inside it is quite restrictive.
PS. I know they're not advertised as waterproof but, if helps, I'd share that I had an extremely miserable night in out in an Ion at the foot of a rock climb in Scotland. Twenty minutes in, my feet were wet. 2 hours in I was soaking wet and by midnight I was soaked soup to nuts and shivering uncontrollably*. Not my best night out. I've only used it as a fair weather bivvy bag or a sleeping bag liner since (for which I find it to be excellent).
(*To be fair to ME, it's only advertised as water resistant. Solely a user error
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Post edited at 20:21