they make the best 4 x 4 box with 2 foot sides.
come in many colors, light weight entire box can be picked up with one hand.
constructed with a box blade and duct tape ( you can color coordinate if you want)
easy to clean, easy to dismantle flat for storage and when needed buy more sheets.
total cost of each box.....$50!! My girls don't seem to mind that I am not spending $1000 on frame.
My box is wood with about 10 coats of paint on it and I've been looking into building a new one too. My (architect) brother suggested using Azek board. Believe it is a solid PVC product. My box is 4'x6' with the ability to drop in a board to make it 4x4 while the pig rail is in it. One of the 4' sides has a hinged drop down so the mama can get in. The corners have two 4" hinges on the outside so when I take it down, I just have to pop the hinge pins, take it apart, and everything stores flat. I have pictures of it and its components if you would like me to share them with you. Just put a note in the subject line so I can grab it out of my spam filter!
Saw these at Potomac and really liked them. No screws, all fit together, easy to clean. Pricey...but I think it's on my list for when my Durawhelp gives out. I really liked it. 4 pieces plus the door and pig rails. Probably still as heavy as a wood box. If I wait for my husband to make mine....I'd be waiting forever. His project list is too long!