I believe the straightforward answer is to dig a hole. But it would certainly be interesting to calculate exactly how much cooling capacity you could get out of

I believe the straightforward answer is to dig a hole. But it would certainly be interesting to calculate exactly how much cooling capacity you could get out of one or two 55 gallon drums of some endothermic precursors. Bonus points for being easily reversible when the electricity comes back on (like those sodium acetate heating packs).