On mine the gap was very small, maybe a 1/16th of an inch, I just used silicone to attach the collar and seal the gap. No issues at all. I also sealed both layers of foam that I insulated with to the collar and the freezer using silicone becuase overkill is fun. ;-)
Thanks Quaffer, you inspired me! Thanks for everything in this thread. Here is the finished unit.
I also mounted a controller in the space of the old one using a wall plate. This way there are no live connections in the freezer itself.
The whole write-up and pictures are here on my...