They do not need to stay that cool at all. When used for solar, the C rate for charging usually never exceeds .2C for overpaneled systems. And usually never goes beyond .5C for discharge. So the heat generated is very small. Now when I am pushing 30-100C rates with a lithium polymer, cooling is an issue. this is why nissan leaf cells were recalled. The battery size was too small for the application, and they overheated which caused significant degradation rates. This is not an issue for solar. Just keep them at room temperature and they are fine.

You can just dig a big hole for them, the earth stays cool enough when you go a meter or two down.

Unless you live in the desert, having the batteries too hot isn't as big of an issue as freezing them.

A $2 circulation pump and an outdoor radiator...

It's good practice to snake a water pipe around every lithium cell anyway in big packs, because otherwise one bad cell can self-heat to the point of catching fire, even while other nearby cells are cold. The flowing water means the bad cell can't get to the point of fire.