For 20x20 mm square pipes. Scale proportional for other dimensions. I may add more sizes. Included variants for horizontal / vertical attachment of accessories (with M5 screw hole). You could remix with third party attachments via slicer (place suitable models overlapping on main clamp).
The bolts need some force to snap into the brackets

PETG recommended with no supports and 6 perimeters
Print in the given orientation for best stability!
my other models for tube / pipe construction
The author marked this model as their own original creation.