I put a 90* bracket onto the inside corners of the collar and secured them with screws. Then I attached the collar to the freezer by gluing it down with construction adhesive. It is the caulk tube type from Home Depot. I can't remember the brand but it's the same adhesive that several other keezers on this site were built with.
Once the collar was on, I used the same glue to attach the foam insulation. Then I used expanding spray foam for any gaps, as well as the foil tape.
The wood you see in the final photos is okoume marine grade plywood. It is about 1/8" thick and wraps around the collar, overhanging the bottom.