I use NIC panels from Walmart. I think both my pens are constructed using 2 packs which at 20$ each turned into 40$ plus a 3$ bag of super cable ties! Now, Im no expert in constructing these things and besides I will be re-modelling them soon when I move but it keeps them in, as well as serves as their 'cages'. I had no idea how to put in a door so I step into them. This doesnt freak them out as you might think it would, thank goodness!