This summer, because of record-breaking heat wave, I couldn't wear kimono at all. Instead, I wore a fan(s) shirt which send wind to your back directly and cool off.
here. On the right shoulder of the page, you will find "see in English" link.
It is a good idea to remove fans from the shirt and put these fans inside of obi. Since they are removable and the shirt is washable. You also need to connect battery box to fans but it is small enough to insert obi.
I did so last summer, since it was only around 30 Celsius, but this year, the temperature almost hit 40C ( 100F)...omb! (Oh my Buddha!)
But my kimono mate Rie was different. she said she wore kimonos in this summer for her health, since kimono protect her from artificial air-conditioning. She must be right. I will wear kimono next summer, for sure, if it is not so hot...
How she looked cool! in Akagi tsumugi with specially twisted weaving and Yaeyama ramie sash. Both were light, thin and cool.