Thanks!
No, I haven't tried preserving brains in a creek.... Have you?
My first thought is predators, unless your always near...
Seems completely do-able, if done right...
I'm sure a cold pit in winter may work, with some cold rocks as the base. In full shade...The average constant temp of the ground is mid-fifties. But I haven't tried with brains.
I have made ground coolers, and they kept the beer and eggs cool!

All it requires is a north side of a structure (house, tree, rock, etc.)in full shade, hole in the ground, and air passage from under and up, to create a 'venturri effect,' stays in the low-mid 50's.
But again, I consider predators. In bear country, the smell of blood carries. Even prey may scatter... Hmmmm....