I don't have the most efficient gas boiler, so I'd like to reclaim some of the heat going up the flue: six feet of 6" galvy before entering a thimble for a tile-lined chimney. I was over at another house today and was impressed as to how efficient the four year old boiler was: you could lay your hand on the PVC stack that exhausted out the side. My 32 y.o. Vermont Castings "Resolute" is the primary heat source, with the gas furnace only coming on when I get lazy....

I've searched the web for heatsinks and don't want any of the so-called 'heat reclaimers' that fit into the flue pipe. Rather, some kind of fluted/folded sheet metal fins that could be strapped to the outside. Since this is a gas boiler, I'm not concerned about creosote build-up that would accompany cooling wood smoke.