I had the exact project on the bench some months ago, and have made a prototype.( I can't go too fast, the typing hurts my brain,from NYE festivities, 3.00pm 1/1 local time)
A 10mm (3/8") copper coil rolled around 2" steel pipe, has baffles in between to slow the coolant down, and is encased in a vessel made of 2"OD copper pipe. Thats the basic thing.
I made this and took some pics as I went, but this was before my digi-camera, so I'll get some scanned, and post them soon. I then encased the vessel in 90mm stormwater plastic pipe (with end caps) and filled between the two with an expanding foam sealant, for insulation.
I am a Plumber/Gasfitter, so this stuff was laying around in my garage, just waiting to be used.. <img src="/forums/images/graemlins/cheers.gif" alt="" />