Water thermal mass wall

Water holds 62.4 Btu's per cubic foot compared to 18 for concrete.  That's a 350% increase.  Water's density is also protection against gamma radiation which is one reason why nuclear reactors are immersed in water.  Simple milk jugs fit well inside the wall cavity.  They can be surrounded my sand to distribute and equalize  the weight.  Coupled with a nearby rocket stove massive Btu storage can happen in a short interval of time