I made mine with 2X2's, 1X10's, and closet rods. I think you can see how it's made at the following picture link: http://im1.shutterfly.com/procserv/47b7dd00b3127cce823ac76d829500000016108QZsXDFu3aa
It's not terribly cheap with the high cost of lumber, but it is very easy to put together.
Hard to get pig rails in a kiddy pool..you could use that later on. That box pic is great! Xpen around the outside should be nice and tight to the box so no one gets out.
We use shelving board that has a formica feel to it (easy to clean), and the boards are 12" high. Then we attach 3" PVC around the middle all the way around with PVC elbows in the corners. Here is a picture of ours.
http://www.bbflabradors.com/Just%20born4.jpg
It is very easy to make and take apart and store when not in use. I have seen a pup move all the way around the back of the bitch while she was laying up against the wall, and never touched the pup. The 3" PVC provides ample room for the pups to stay safe.
It is attached with screws. I love it. I have checked on mom and pups several times and found mom laying up against the PVC and a pup safely underneath it. If the PVC hadn't been there, they may have been laid on.