I asked this question in a different thread and didn't get an answer. When I looked through Rafal's build threads and templates he mentioned 2mm cardboard. I looked around on Amazon for something but I couldn't find it in 2mm sizes. Based on mm to inch conversion I think this equates to the .08" thick cardboard or chipboard. I bought that and some thinner stuff that I'm going to play around with and I"ll report back.
I've seen people use everything from whatever cardboard they could find like cereal boxes to using plastic signs etc.