One thing I might recommend is different size PVC for the rails. I use LARGE PVC so that it is not flexible. What you have set up there would not work for me at all.
You can see in these photos the rails I use. This is heavy enough to stay put, and it fits tightly inside my box. My whelping box set up is a little over the top, but maybe you will forgive it, and hope you can see the pig rails. They fit right up against the walls of the box, and they are about 5" from the floor at the bottom.
So very sorry you lost a puppy, that is very painful every time. 

