Halite melts at 801 °C, 1074 K, 1474 °F, is that tough enough?
If so, I see just hole-sawing an "L" shaped passages through two blocks, butting them against each other and packing the whole thing in clay/ perlite.
A riser could be stacked salt blocks with appropriately sized holes bored strait through. More clay/ perlite packed around as insulation,in a drum or pipe.
I have not had any luck finding an R value for rock salt and the thermal conductivity values are beyond my ken at this point.
William Bronson : They are making Funeral urns out of blocks of salt (in india)for people who want to be buried at sea, One of the time honored ways of getting dairy cattle to get their minerals, has been in salt blocks this of course gets them to drink lots of water to encourage milk production, these have always had mold marks on them, and
I remember them as quite heavy, so I expect that you would need to insulate on the inside but then again 'till its been tried we don't know what other laws of physics might
apply ! If its good enough for Permies- Think like fire, flow like a gas, Don't be the Marshmallow! Your comments/questions are solicited and Welcome ! PYRO - Big AL
Success has a Thousand Fathers , Failure is an Orphan
Salt doesn't like water or thermal shock. When cold, crystaline salt is brittle. I suspect that it would absorb moisture from the room during the summer when not in use. A bloom often occurs on salt blocks during humid weather.
Check out salt glazing, to see what happens when it is super heated.
There are some salts that store more heat than other substances. In very small installations where space is at a premium or weight is a concern, these phase change salts might be useful. Cob is cheaper.