From Ryan’s assembly instructions: C-23.5mm, F-25mm, or J-25.4mm (1in). The measurement is for the Outside Diameter of the conduit/rails/tubing.
The answer is no for 27mm. That being said, unless you want to recreate your printed parts (I wouldn’t encourage that) you’ll have to search for smaller OD material that equals the three referenced sizes above. Check AMAZON.de for tubing. This link is for the Netherlands only.