Does anyone have any suggestions as to what I can use for pig rails in my whelping box? I have tried plastic pipe before but it is too bendy ( I can't get a good fixing for a support halfway along as the sides are MDF) I wondered about broom sticks but wanted something a bit cheaper really as I will need four.

Here are some sites that have different styles of home made whelping boxes. Didn't you read up on how to make them before your dog was bread?

How to  Build a Dog Kennel Complex, Dog House, a Whelping Box, Agility Equipment, Rabbit Hutch, Aviary, Cat Cage ... and WHERE TO GET SHOW TROLLEYS from Chinaroad Lowchens of Australia (http://www.lowchensaustralia.com/diy/whelpingbox.htm)

Whelping Boxes (http://www.breedingdogsguidebook.com/recos/whelping-box.html)

Whelping Box Construction Plans (http://www.mcemn.com/WB-WhelpingBox.html)

We always just used strips of wood from the hardware store.
Ends up costing about $5.
I wish I had pictures of the old box but it got thrown away.

I did a quick Google and this is pretty close to what ours was like:
http://www.flashbackfcr.com/Snicker/July-3-whelping-box.jpg