I was originally searching for an odd end of large-diameter piping from a building suppliers, but by chance got that big lump of cardboard from a used hose reel in the same place. The diameter of that was a little larger than I wanted, but take a look in your local DIY place or builder's skips for some ideas (although there's not much building going on, right? <!-- s:) --><img src="{SMILIES_PATH}/icon_smile.gif" alt="

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On the first one n1mbus and I made, we used a large paint tin (the 5 litre jobbies) which was a perfect diameter for the boiler we has at the time, as it needed to be taller rather than wider. Basically, try to keep as much of the coil near the top of your wort for best cooling (heat rises and all that).