How can you inhibit crystallisation of hot superconcentrated magnesium sulphate
First, your question involves inorganic chemistry, not organic, unless you're talking about the unmentioned gelling agent.

Second: Impurities serve as a source of nucleation, and stirring increases crystallization. If you leave the supersaturated solution in a clean container and allow it to cool slowly, e.g. in a Styrofoam cooler chest, it may take a long time to form crystals. If you use some thickener, such as PEG or gelatin, it might take longer yet. Left undisturbed, though, supersaturated $\ce{MgSO4}$ can persist for years. There are videos of it crystallizing after a seed is added.