What is a tube? Tubes are a cylindrical form that is measured by its inside radius, the basic concept is not what the tube is, but what can you create with this tube?
Think of an idea, we can 3d print the hardware that will hold your creation together.
You can build a bicycle:
Of course, the stability of the skeleton it dependent on the material https://theproductlaunchcompany.com/blogs/news/about-materials, You should try our do it yourself kits. It's a Great idea for family summer projects.