John, a tubular type core can probably be adapted to your tanks with strips of copper soldered into the groove in the upper and lower header, and then shaped to fit the tanks. Since there won't be any pressure to deal with, the solder joints won't need to be super strong, just leakproof.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!