I bought three or maybe four sheets of rigid foam insulation at whatever thickness matched the width of the c-channels in my garage door panels. Think it cost $100 and took about a day. They were a surprising pain in the ass to cut, though. A heat-knife with a really long blade would probably be ideal. After hand cutting two sections, I just ran the rest through my table saw. Foam snow everywhere.

I don't know that I have a direct shot of my garage door, but it's in the background of a lot of my project pictures.