In reply to Tall Clare:
We did this:
base section waterproof outer, pile liner and karrimat inbetween.
upper section gortex ['cos we happend to have some], lined with pile. For your dog, use a synthetic filling.
Critical design feature: top and bottom fixed at the back, x2 side zips, open front.
Unzip, dog instructed to lie down. Throw top over dog, zip up. Dog is not too confined and can, with a bit of practise, turn round during the night without coming out of the bag. Too hot? Lie on top.
In snow holes etc we also threw out duvet jackets on top