The University of Minnesota has advice for you!

https://extension.umn.edu/preserving-and-preparing/power-out...

The tl;dr:

• Having a fridge thermometer & a freezer thermometer are good.

• A full freezer, kept closed, can hold temperature for ≤48 hours. A half-full freezer, ≤24 hours. Fill your freezer. Ice (to fill up empty space) is your friend. You can make it, or buy it from the store.

• The fridge part, kept closed, can hold temperatures for ≤4 hours. Block ice can help extend that time. A fridge thermometer can be used (after power is back) to check if the temperature exceeded 40° F.

Block ice? It's exactly that, ice in the form of a large block. https://elitecooler.com/blogs/news/cooler-use-get-the-best-i... has more info on using it for keeping things cool.