At first we used a plate heat exchanger but they are a pain to clean and not very effective so we made our own counterflow cooler out of 9 m of 12 mm copper tubing, some reinforced PVC hose and some pipe fittings. We wrapped grass trimmer cord around the pipe to delaminate the flow of cooling water.

At max transfer flow rate it is capable of cooling 80C wort down to 20C without breaking a sweat!

Update

The DIY cooler is now replaced with a stainless cooler from Brew Monk.