I see two major problems with the initial approach:

• wind load
• not really suited for larger solar panels - due to larger wind load, shadowing of windows below etc

I thought that it might be a good idea to mount a larger panel vertically leaned against the wall below the window. I hope this lowers the load on the telescopic rod holding everything and lowers the wind load siginificantly. The lower efficiency would more than compensated by the use of a larger panel (that doesn't cost that much more).
I had a few ideas that I illustrated:

All these are side views. A is the basic concept. 1 Is a 30 mm aluminium strip with 2 mm thickness that connects the telescopic rod 2 (clamped into the window frame) and the solar panel 3. Two are needed to mount the solar panel. 4 is a spacer between the wall and the aluminium mount. The weight of the panel rests on the telescopic rod.

In B, the aluminium strip rests on the window sill to lower the load on the rod. The problem is that the angle of the window sill is not standardized. 5° are recommended, but much more can be common.

Approach C adds an instrument foot with integrated ball joint to the strip to make the design fit varous angles of window sills. Approach D has an additional strip that has a variable angle and can be adjusted with a setscrew.

In E, the telesopic rod terminates in wall pads with a flat underside that rest on the window sill. This would be a simple approach to lower the load on the bar, but I am afraid that tensioning the bar would actually prevent the whole construction of resting on the sill.

For simplicity, I'm torn between trying approach B for the angle of my window sill or E for flexibility.