Sand doesn't hold heat very well. I'd get straw or pine bedding instead.
Second, I'd remove the plastic. Ventilation is very important.
https://www.backyardchickens.com/ar...-there-and-cut-more-holes-in-your-coop.47774/
Third, check for drafts. You don't want any entrances facing the wind, and you don't want any ventilation holes directly across from each other. You want to achieve the happy medium between too much air movement (no chance for the ducks to warm up the air around them) and too little (ammonia and moisture buildup)
Fourth, put a sheltered area behind your coop so that you can keep the water outside. (If you haven't already done that) Water in the coop is a bad thing in winter.
And don't worry about it too much. Ducks are half-feathers (seriously; try plucking a duck carcass. They're equipped with the equivalent of two heavy winter coats.) Mine are perfectly happy at zero Fahrenheit or lower, so long as they have a nice, dry floor and a windblock. And lots of food, the feathered pigs.