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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.
Have families donate PVC pipes, elbows, 3-joints, etc. Cut pipes to various lengths.This is great to have in building station.Children can work together to create cubbies, etc. Great way to learn concepts of math, use of physical space, problem solving strategies, etc. Can use scarves to cover structure.