The connectors are designed to be used in combination with 20mm outer diameter PVC pipes ("Isolierrohr" or "ISO-Rohr") which are very cheap and easy available in home improvement stores in Germany (e.g. "Kopp Isolierrohr" from Globus Baumarkt).
By cutting the pieces from pipes in desired lengths, you can create different kind of frames (e.g. for a 3D-printer housing, spool holdes etc.).
The rounded pieces can be used as feet if used in an angle (e.g. in a spool holder https://www.thingiverse.com/thing:3507395)
AIO Robotics PLA
Cura Settings (just for reference):
Wall Thickness: 2mm
Top/Bottom Thicknes: 1.6mm
Infill Pattern: Cubic
Support Placement: Touching Buildplate
After printing and removing the support, it is recommended to treat the supported areas with sandpaper to avoid marks in the PVC pipe.