I want to be able to erect arbitrarily shaped structures that, for example, could support the weight of a person doing pull-ups or support several adults sitting on it if formed into a box shape or whatever. But I also want to be able to put it together quickly, without need for glue, hammer, screws, or anything. And I would like to be able to take it apart and pack it flat for moving around to different locations or put it in my closet.
Is there any good materials for this? I went through Home Depot and 3/4" steel pipe seems somewhat reasonable, but there don’t seem to be any 45 degree angle fittings, which seems like it would be necessary to triangulate the faces of any shapes, for structural integrity.