I bought the same freezer this summer from a guy on Craigslist for $75, was almost brand new. I'm so lazy, I finally got around to building a collar. I like how mine turned out, but I built it on the top outside rim, and I'm not sure how to secure the collar to the freezer. I was going to just use a large bead of caulk, but I'm not sure if it will hold.
I used an old 2x4 that was in my garage and cut and stained it, I'm horrible with wood work, but it turned out pretty well without doing miter cuts.