Many thanks for the suggestions. I will go for the plastic tubes and joints, but I will not abandon my alu-profile ideas. I will try to build elements out of tubing which can be combined (just like lego) and build softboxes of different size and shape. I think that alu-profiles and tubing might be better for such a softbox system.
As expected, I am not the first person building it's own softboxes. I was hoping that someone could point me to some descriptions, drawings and diagrams on the web, so that I don't have to spend too much time with the design work.
You might ask - what design work? Well I hope to show you soon.