when I used swimming pools like that for a brooder I got a second one and cut the bottom out maybe leaving six inches of the bottom in a ring... Then C-Clamped the two to gether... like a Clam... Now you can put window screen on the top and it will contain the little guys...
Oh heck like this:
instead of C clamps you could also use WIdE duct tape.... but the clamps allow you to clean out
Not sure in anyone said this already but you can give chicks some warm, hard boiled egg yolk. Some of them go nuts over it and it gives them a nutritional boost.