This is a standard version of the square pipe connector. This is different from my earlier model ( https://www.thingiverse.com/thing:4805792) . I'm afraid to say, instead of being short, printing requires support.

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 side length

[Customizable parameters]
-pipe inner side (min:8mm at M3, max: no limit)
-pipe outer side
-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.
-This inner is exactly the same as the inner of a square pipe "cap" if the cut angle is the same.

[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]
2021/04/08
-Changed the cut angle from 30 degrees to 45 degrees. The angle used for the cap is 30 degrees, but I thought that the connector needed a little more strength, so I changed it to 45 degrees.
-The hole diameter for bolts has been increased a little.

-Please refer to the following video for changing the parameters of fusion360.
This video is for round pipe cap, but you can understand the way to change parameters.