I have a small restaurant 1-door (vertical) freezer that I converted to a keezer for my bar setup. It was easy to do, essentially extending out the cooler space horizontally.
Depending on the setup of the bottom of the freezer, where legs are, if any, you could attach brackets there and then that will give you proper support for the bottom piece of the collar. Since gravity will be your enemy here, just brainstorm some ideas on how to support the vertical collar (in addition to using epoxy/glue to secure collar to freezer opening). Thinking more into it, just build supports beneath the bottom piece of the collar, that way when you re-attach the door, all the weight should be supported.