I wouldn't put the rubble sack over, it might cause rotting. Horticultural fleece is the usual wrapping material, but even that should be removed when the weather has milder spells during winter. You could just pop your rubble sack over at night if the weather gets bitter cold, and remove it as soon as possible every day, replacing the next night if necessary. Most years Cordylines survive the winter pretty well, it was only last year that caused a huge number of losses, but you haven't made public what part of the country you live in - were you down South, I'd not worry too much, just keep it tied up.