Like others have said, you need to remove the seal if it is attached to the freezer itself and not the lid. Mine was attached to the lid, so it was not a problem.
There are many ways to attach a collar and I chose the easiest - I ran a heavy silicone bead around the perimeter and set the collar on top. I then sealed around the inside and outside with another bead that I smoothed with my finger.
It's been together almost a year and I've never had a problem. Good luck with your project - it is a lot of fun to build one of these.