When the school I teach at finished a new campus and moved into it, I saved as many of the large cardboard tubes from flooring that I could. I'm an art teacher and I always try to save good junk, even if I don't know what I'll use it for. The last time our school moved, more than ten years ago, I saved some of the carpet tubes and made a jungle in one of the hallways. I had similar plans for these tubes, but my wife intervened. She is the principal of our Early Childhood Center (age 3 through kindergarten) and wanted a set of giant Lincoln Logs for the kids to build with. After spending some time searching online for a free plan or tutorial on how to make them and not finding one, I sat down and figured it out myself, first with paper towel tubes and then with the flooring tubes. The tubes I used mostly came from a linoleum-type flooring, but any large tubes would work, I think, probably even PVC pipes. PVC pipes would be good for an outdoor fort, whereas carpet tubes would obviously last longer indoors than out. Fortunately, this project was designed to be used in an indoor playroom.
I am a teacher, and my job is to share knowledge and teach skills to others. I get annoyed when others won't freely do that also, and so I am now going to freely share with you what I have learned the long way. I hope it will save you some time and that you'll have fun with this project.