For a recent camping trip I had to get a large cooler at short notice. As large coolers are expensive and I didn't know if I'd ever need it again, I decided to upcycle an old fishmongers box.

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

Once you have your soon-to-be cooler, give it thorough clean to remove most of the fishy smell and any bacteria that may be lurking...

Step 2: Step 2 - Make a Drain Plug or Line the Box With Something Waterproof

The box will have drain holes in it (in this case four) you can either plug them, 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.

Step 3: Step 3 - Fill With Ice and Enjoy

... and there you have it... a finished cool box ready to be filled with ice and beer...

In hindsight a lid-securing mechanism was also required, so go get some bungees or ratchet straps...