I have a similar-sized keezer. No plans. I just carefully cut 2x6's to fit the opening, made sure they were perfect, then screwed them together. Position the freezer lid on the top, carefully align the hinges, and attach.
I did something somewhat different than most people--I didn't glue down the 2x6's to the freezer. Instead, I used pickup truck
topper tape (what you put under a topper to cushion it against the sides of the bed) on the top of the freezer, then set the 2x6 box on top of that to seal it to the freezer.
What keeps it in place is once I had that positioned, I attached a 1x8" facade to it to hold it in place. I have that on the front and both sides. Here are a few pics:
In the first pic, you can see the mouth of the freezer isn't the same level all along--the sides of the mouth stand a little proud. This would be a problem if you were to permanently attach the collar, you'd have to fill it with something (probably silicone caulk). Can be done, of course.
In my case, I used some thin plastic strip and double-sided tape to build up the slightly sunken part so it was level all the way around. I then added the truck-bed tape and set the 2x6 collar on it.
If you look carefully at the second pic, you'll see the facade attached. It's only on the front and both sides; I left it off the back so I could attach the hinges. it's plenty heavy enough to seal against the topper tape, and here's the beauty of it:
I can remove the collar and, if I want, I can return the freezer to being just a freezer. It also makes it far easier to move. Mine's in my basement, and it was hard enough to move just the base part w/o the collar. The collar and lid just come off.
If I were to build another keezer, and that's not outside the realm of possibility, I would do it exactly the same way.
Here's the finished product:
As one final note: This freezer was used and had dings all over it. You can see that in the first two pics. By painting it with chalkboard paint it camouflaged all those imperfections and gives me a way to put a menu on it.