My wife and I put on a wedding party for some close friends. We had never had a party at our house for more than 60 people. So besides constructing a 60 square foot buffet table in our garden. I also wanted a large cooler for soft drinks and beer. A trip to our local big box store netted me: 1 2'x4' sheet of 1' rigid foam, a 4'x8' sheet of 2" rigid foam and seven 8' 2x4's (mystery "white" wood). If I was building this cooler to be a permanent outdoor fixture I would have used redwood or cedar. I iced 5 cases of beverages with plenty of room to spare. This design is scalable to whatever size cooler you need.
Start by assembling the base and top, they are identical with the exception of the bottom having a brace in the middle. The inside dimension is 24" x 32.5". Basically the frame is just 2x4's screwed together with 3 inch deck screws at right angles.
Step 2: Place the Rigid Foam
My foam pieces: the bottom is 1' foam 24" x 44.5", sides are 2" foam 18" x 44.5", ends are 2" foam 18" x 20". everything should fit tightly together. I had an extra piece of 24" x 48" x 2" foam I used for a loose top.