PVC by It's Great To Be Home. This blogger posted all the plans to make this. It only takes one person and can be done in an afternoon for aprox. $60. SHe is planning to do a follow up post with the cover for the fort. I think sewing a fabric cover could be fabulous. That would keep it light weight and easy to move around if needed.
Free PVC playhouse pictures and plans--ok,this tutorial is for a playhouse which gets covered with a fabric tent, but....I was thinking why not cover with plastic and make a greenhouse? I have seen clips made from pvc pieces that are sawed part way through that you use to clip over the plastic onto the pvc pipe frame. What do you think?