While checking out the Herlt boiler site I noticed their storage system which looks like a big foil bag in a sort of spindly looking wire cage, and sheep's wool insulation. Given the rather low tech look of this setup I can't imagine it would be terribly costly and may be able to serve as some inspiration for the DIY and experimental crowd here. I was trying to find any importers of the Herlt line in North America and found this link but it's from 2004 so I'm not sure if that Canadian company is importing them. The same company appears to also have been a dsitributor for Atmos boilers. I threw a couple emails out there and I'll see what comes back.