Karen, would a rubber cow/stable mat do in an un-floored ark?
It would definately be better than nothing.
I'd try to have it so the mat came up the inside of the arc slightly to form a barrier to any water/snow/ice on the outside (if that makes sense) if you're really worried about them being cold a layer of shavings under the straw really helps the 'cosiness' factor.
But, if they've got a bit of fat (and what Kune doesn't
) and a buddy to snuggle up with they should be fine.
A wee bit of fun (depending on how mad you are
) is to go out when it's snowing and throw some of their favourite veg on the ground - they spend hours digging through the snow to find it
HTH
Karen x