I'd look for repair drawings for the make/model freezer. That will show you where the coils are located. Once you know that, you can plan the hole placement.
Personally, I think the collar idea is a sound one. Even if you just use some 2x4's to do it. Then get one of the gas bulkheads (Austin sells them) to send the CO2 into the keezer. That way, you don't need to worry about sealing around the hose. Plus, it gives you other options. Like connecting a ball lock post to either side for easy connect/disconnect of the CO2 without needing a wrench.
I'm looking at doing that with at least one of the CO2 feeds going into my brew fridge.