To obtain the box I went round my local supermarket fish counters until one kindly gave me a box, in this case from a recent delivery of Salmon.
Step 1: Step 1 - A thorough clean
Step 2: Step 2 - Make a drain plug or line the box with something waterproof
Ideally a cooler should have a removable drain plug at the bottom so you can drain water off as ice melts.
I used builders DPM (Damp Proof Membrane) as I didn't have time to make removable plugs.
I folded the DPM into the box, cut it to shape and then taped the joins so they didn't unfold.