My collar is a force fitted on. I cut the pieces of wood just slightly larger than what I measured, and then sanded them down until they pressed into place. Then just bracketed them together, and used silicone to seal it all.
The inner collar fits into a slight depression in the top edge and two of the outer four are on the outside of the wall and extend about 1/2 inch below the edge. The only actual attachments are the hinges for the lid. One screw for each hinge goes into the back of the freezer, the other into the collar.
Filling the space between the collar walls with latex insulating foam gives it an air-tight seal, top & bottom. If I was doing it over again, I'd put braces between the walls above each shank hole. There's a little too much flex and the taps tend to rotate.