How to store smoked fish when traveling?
Are you sure it was fungus, if it was just a dust? I don't know what else it would be, but 8 hours seems short for that to grow.

In any case. Keep it very dry, perhaps by wrapping in parchment or muslin and packing in rock salt before sealing all in plastic. Cool it with ice, sealed in a separate plastic bag, or preferably gel freezer packs, also separate.

I've never been in that situation, so I can't say I've tested this advice.