Use a block of solid (pure) copper
The only other "readily available" material with similar thermal conductivity is pure Silver
Unless one can find somebody to cut a Diamond with the right dimensions... or can "grow" a block of graphene to the right specs... filling a cube with liquid helium is just too cumbersome. And water is just not a good thermal conductor.
But if one is willing to settle with something with less conductive properties as Copper, and more as water.. there are blocks of Iron, Aluminium, Steel, Brass, Salt (wet or dry), Lead, Clay, Soil (wet/dry) , ....
Either way , all these pelts need to bolted to solid blocks of metal
The only thing i am not sure of, is if this design is not breaking some laws of physics , but good luck anyways