Cheap robust (8Eur per piece) L-Type shelf brackets (30x45x3cm) were slightly modified.

The last few centimeters of the long arm were bent with the help of a vice and a hammer:

...one additional hole per bracket with the right distance makes the parts fit:

The whole construction fits nicely on the outside of my balcony railing. Two original mounting screws for the railing were replaced with longer ones, now holding the solar panel in place, too.

The mounting angle is 45°-ish (30° would be better for overall collected energy). This means collected energy in summer when sun is high is not as good, but in the other seasons where the sun is located lower, more energy is collected, plus leafes or snow have a higher chance to get off the panel.