Height is also an issue to consider. Without the collar there would be little or no room in my chest keezer for the lines and gas manifolds.
I used some cheap weatherstripping, and get a fine seal. Mine is just cedar 2/4s. I picked through a while to find straight grained ones, and they have not warped. I made mine with mitered and chamfered corners, but that's because another hobby of mine is woodworking. It doesn't work any better that way than if I had cut it with a handsaw and nailed it together, it just wins me bragging rights -- until people look at some of the work done and shown here on HBT!