Throw it in a Thermoplastic Polyurethane (TPU) bag and it'll be a pretty good long term solution.

> First, in a watertight plastic bag and then in the water?

Was wondering, but this the most duct tap hacker solution!

As I discovered cooling down hardboiled eggs, it's better to keep a thin layer of moisture that cools the object via evaporation.

Something of this sort should keep the device moisturised:

https://www.thehydrobros.com/products/automatic-water-spraye...

0.2ml/s at its lowest setting looks like the ballpark of what's required to maintain temperature.

a sandwich bag would work wonders, then you could use ice to counter the plastic's thermal inefficiencies!