The reflective fabric we used is very lightweight, strong, and dries quickly after rain. The clamps for attaching the guy ropes come from a Chinese website. Unfortunately, those clamps (middle photo) don't work in practice. Therefore, we hammered in eyelets at the corners of the screens. The shades can be tensioned at an angle, but with poles, they become more like little awnings.

In practice, it works well. The material reflects the sun, keeping it extra cool. You could also sew elastic or Velcro straps to the bottom corners so the screens can be secured to the windows. But by securing it separately with a guy rope and a peg, it's more independent of the window's position.

Comments and questions:

Pimpernel: But sun protection fabric is not waterproof? Answer: We discovered that too, Pimpernel. But that's not a problem for this application. The advantage of this fabric is that it's very strong and super thin: it weighs virtually nothing... And it reflects sunlight perfectly.