A long full skirt - either panelled or tiered with elasticated waist, or semi-elasticated waist, boho / ethnic style but not too full on.
AND in 100% cotton or other natural fabric - not polyester or rayon etc. And not too expensive - I haven't got much money to spare at the moment but I realised yesterday that I have almost zero summer clothes and I find a lot of the manmade fabrics far too hot ...