Consider something like this - cheap, easy- also available at ACE HW
stores. It will fill the gaps around the door.
http://www.hardwareandtools.com/invt/6540116#



T. T. wrote:
Dear all:

Just a quick question wish you could help. We are living in a very old
house (1930's) which has a old wooden door that has a 5" x 8" (outside
rim: 7" x 12") viewing windows (or maybe I should say "door")... This
viewing "window" is quite old and is not sealed very well, so when we
turn on the heater, the warm air will escape from this viewing window -
not really energy-smart. As I just had a good luck ordering a
hard-to-find window part from a online store, I wonder if there is also
an online store that sell this kind of part, and if so, where could I
find it? And what is the official name of this type of part?? Many
thanks!

Here you could see some photos of this "door" that I'm talking about...

http://www.picturetrail.com/hgtv2006/
(click on What is THIS?)

Thanks!