This is a connector for round pipe. It can be applied to pipes of various diameters.

I made two styles of connectors.
One is the cube shape. (https://www.thingiverse.com/thing:4818997)
The other is the spherical shape of this time.

There are two types, "2 direction type" and "3 direction type".　It can handle various pipe size. Customization is done by changing the parameters of fusion360 file (.f3d).
Try this connector once.

[Features]
-Customizable
-Fixed firmly
-Hard to break even if squeezed strongly with a wrench
-Easy to remove from the pipe

[Design point]
-Designed so that when the bolt is tightened, the bolt is parallel to the pipe. This enabled stable tightening.

[Bolt/Nut type to use]
type: Countersunk screw bolt and Nut M3 - M6 (If the screw heads do not hit each other, you can use screws of other shapes.)
bolt length: approximately equal to or longer than the pipe diameter.

[Customizable parameters]
-pipe inner diameter (min:8mm at M3)
-pipe outer diameter
-screw size (M3, M4 ,M5, M6)

Detailed screw dimensions are set automatically.
Screws larger than M6 are also possible by manually setting the screw related parameters.

[print conditions]
support: yes
brim: no
filament: PETG, may be PLA, ABS

[note]
-Inner is common to 2 directions and 3 directions.

[caution]
-If you get an error while changing the parameters, click the OK button in the parameter change window, and then click the undo button to return. This is because even if you try to change the parameters without undo to eliminate errors, the abnormal state may be maintained.

[revision]
2021/04/13(v3)
Changed the default cut angle value from 45 degrees to "90-acos(sqrt(2/3))" degrees.

2021/04/10(v2)
I reviewed the design a little. There is almost no change in appearance.

Currently, the cut angle is set to "90-acos(sqrt(2/3)) = 54.73561degrees", so you can print in the posture shown in the figure below. Therefore, the support material can be minimized. Furthermore, the appearance of the connector can be kept clean and the strengths in the three directions are the same.
However, in this case, the length of the arm is a little longer, so if a shorter arm length is required, I think that it should be considered to set the cut angle to 45 degrees or 30 degrees.