When you mentioned kiddie pool, I just remembered seeing something at Home Depot today that might work well for you instead. In the concrete section of the store they have these plastic mixing tubs that are made to mix mortar or concrete in them with a shovel. They had two sizes.
Here, just did a google and found them. Big one is 2'x3'. They might be higher cost than kiddie pool, but they would be pretty tough I would think.
concrete mixing tubs What I did was use my farm tractor and loader and just dumped rock piles, then smaller rock on top. But my water was still down a few feet from full pool. So I could run the tractor over the edge, use the bucket on the loader to scrape a dirt ledge, then dump the rock on this ledge. You probably can't do that with a full pond.
You might be able to get into the water and place two concrete blocks (about a buck and a half each) down into the mud, then set the outer edge of your container on these blocks and let the other end rest on the dirt. That would keep the container more level than just trying to let it set on the 3-1 slope.
Edit: oops, just read Cecil's post above about not using square nesting areas. Disregard everything I said.
