I'm a little concerned about the hibernaculum made using the grow bag - it could become totally waterlogged which would not be good. If using this method I'd recommend making several good slits in the bottom of the growbag to avoid it becoming a sump.
Damp is good, totally waterlogged could be tragic.
I agree, it wouldn't take long for a growbag to be full of water, or to freeze solid afterwards
I wondered about that too. Would have thought just putting a good layer of compost on soil and then covering etc would have been well enough insulated but cool and damp. With the amount of rainfall we normally get here they'd need a raft by December!