Building and insulating a collar is one of the easiest ways to build a system. And you can make it as expensive or as cheap as you want. Check out some of the other threads (here's a good one
) for some inspiration and visuals. The easiest way to do it IMO is a square-cut collar glued directly to the keezer with foam insulation on the inside.
Measure out what you need, head to Home Depot or Lowes and pick out some wood that's the right height. Depending on your budget and how much time you want to spend working on it you can pick up some decent hardwood, stain, polyurethane, fine sandpaper etc. Or just get some cheap pine, it's really up to you. Have them cut the wood for you and pick up some screws, angle brackets, and a big sheet of insulation. Remove the freezer's lid (with hinges), make sure the wood's the right size, drill holes for shanks, then screw the collar together and reinforce the inside with L-brackets. Cut the foam insulation to size and fit that on the inside of the collar. Use silicone caulk to hold the insulation to the collar and the collar to the keezer, also caulk the crap out of all the seams to make sure you have no air leaks.
Optional but highly recommended, especially with a tall collar: mount a small PC fan inside the collar to help keep the air moving, and put an Eva Dry dehumidifier in the keezer as well.
Once the collar's glued on and everything's caulked up, I'd put the lid on top, weigh it down, and give it a good 48+ hours for everything to dry. Then screw the lid hinges into the collar, and you're good to go.