Heat is generally over-kill. The dog will get too hot in there, pull out the bedding, and not go in.
A properly sized dog house uses the dog's body temperature to keep the critter warm. If it is too large, it will not keep him warm. Whatever you hang over the door, may be something that your dog will pull off and possibly ingest.
GSDs like to jump on top of their dog houses and lay on them. Be sure that if it is near a side of the fencing the dog will not be able to use it to get out.
Jenna, RN CGC & Babs, CD RA CGC HIC
Heidi, RA CGC
SG3 Odessa, SchH1, Kkl1, AD
Ninja, RN CGC & Milla, RN CGC
Joy, Star Puppy, RN CGC
Dolly CGC & Bear CGC