The music stand has a triangle shaped metal sheet at the back with no trivial form to fit. Check the original Thomann Orchestra Stand Deluxe. 

Therefore I created this replacement part with the following objections:

The angle is fixed at 20°, because I want a robust solution. A 3D-printed variable angle would add to much complexity or wobbling parts.

The part is designed for usual 22 mm rods. Therefore the hole has a diameter of 23,2 mm. The part can be screwed to the tubing with a M8 screw and nut.

I used 57907-tough-hand-knob-for-m6-and-m8-nuts, as shown in the pictures, with a M8x30 mm hexagon cap screw. Probably 40 mm would be a good length. Because my screws are shorter, I shortened the knob by 8 mm without a problem.

The M8 nut is fixed in a small hexagonal hole in this part against rotation. 

Please feel free to remix the part as needed, with the attached FreeCad Files.