Took this technique I learned in prison to make it bearable during the heatwave:

1. Wet the window with a sponge or rag.
2. Put aluminium foil up against it. Shiny side out. Cut beforehand on angled windows like that or use the roll on vertical surfaces.
3. To cover the overlap, just wet the foil already on there, but be careful not to pull it off at the edge. An overlapping strip will stick to the other ones.
4. Use masking tape to cover any overlapping edges that come away, as well as the edges that have a hard time connecting to the windowsill.

This keeps out an incredible amount of heat; you can feel how hot the foil itself gets during the day, while keeping the inside nice and cool.