The "shelf" idea has been on my mind for a while
Have you ever noticed if you leave them to themselves, they typically WON'T lay on pups?
It's usually when they are in a square, flat bottom, little nesting material, boxes when they lay on one.
I've had litters and pyled in the straw. Deep. They mother would dig out the middle and the pups would all center to the bottom of the nest when she was gone. As early as 1 week old, if she's in the nest, they would travel up to the edges, above her.

Personally, Im with OGDOGG, I think the swim pool is about as good as it gets. And personally, I think anything with a flat bottom with no material for mom to make a nest is the worst.

Thoughts?