i'm very interested in water thermal storage and I think I have a cheap easy solution. What about directing the exhaust through the open holes of a foundation of cinder blocks (two passes for a total of 10 feet). The cinder blocks would be sealed together using high temperature silicone caulking, and would be lifted off of the cement floor by rows of bricks for an air gap. Once this cinderblock heated slab (I'm thinking about 3'X5') is built, I will make a plywood box around it and use plastic sheet, like pond liner, to hold a mass of water about 2 feet deep.
I think I might be able to get away with the plastic sheet directly on the cinder blocks, since the water should strip the heat away very quickly, but if that doesn't work, I could build up a thickness of cob or sand and try the liner again. My basement has a floor drain, so I'm not too worried if there is a leak during R&D.