25mm is also relativly hard to come by in Denmark and northen Germany. For Plumbing we(in Denmark) used the same dimension(26,9 mm) as you do for Scaffolding. It is called 1inch tubing, due to the inner diameter.
It could be very very nice if we somehow could make the whole thing parametric. Width in a certain range say 20-30mm. Perhaps make stl from SW without the holes. The import stl in openscad scale it man the make the holes.
PS. Thanks to Vicious1, I am really enjoy making the project.