@Longpork3@lemmy.nz wrote down the recipe here.

1.2l water

240ml sodium sulfate

60ml sodium chloride

20ml xantham gum(optional for increased efficacy by keeping the solution homogenous)

Boil water, stir until fully dissolved, a small amount of solute should remain, if not, increase sodium sulfate concentration slowly until it does, indicating no free water molecules available for dissolution.

Solution should now be cooled to below 18c( freezing point) for an end product that will regulate temperature to 18c so long as it have sufficient(negative) thermal energy.

Solution of pure sodium chloride will have freezing point approx -20C, while solution of pure sodium sulfate has freezing point +35C. Adjusting the ratio of NaCl to Na2SO4 will shift the freezing point towards either end of thag spectrum, depending on what phase change temperature you are targetting.