Lara,
You know if you really get into a bind.... go to an Army Surplus store & pick up a couple of the green wool blankets. Then experiment. Lining with flannel would keep him warm & wool keeps warm even when wet. It's even naturally water repellent. Being a GSD, it shouldn't be too difficult to try out various shapes :D It's the smaller ones that frustrate me :)
Anyway, just a thought :)
toodles