Hi,
I’m thinking about buying
but I found a video showing that the mounting can be problematic
What is your experience concerning mounting SXTsq?
Thank you 
Once you accept that the mounting is not in the middle of the SXTsq, there is no problem.
One of the 4 sides is zip-tied or metal band tied to a round tube . If there is no mast or pole, then use QM-X, QM, or QME.
QMP comes with 2 connectors, so it can also be used with the second connector …https://i.mt.lv/cdn/rb_images/1445_m.png
The assymetric mounting of the SXTsq is not what you normally expect. For a mechanical engineer this is not the best way to make a stable fixation. (The tension in the metal belt must be high to create enough pressure between SXT and the tube due to the bad pressure angle, and only the friction from this pressure can create the needed torque to stop forces that now have a long lever.) But the SXt is lightweight, and maybe it all can hold its position in the recent 130 km/h wind storms.
If you prefer a more central bracket then maybe one can 3D print the standaard SXT/Omnitik slot. (https://www.thingiverse.com/thing:3062748) .
Anyway a setup where one uses two sides of the SXT connection points, or a part between the SXT and the pipe, can easely create better pressure angles for the belt over the pipe/tube.