I'd say go for more length of fabric and more pleats.

In the heat you waft away hot air as the pleats swing.

The 4 yard box pleat needs a sturdy fabric to give it gravitas, or it might look a bit skirt-ish.

I wear mostly soft cotton fabric in the summer, as I feel the heat more than most people being from the cool and mountainous north of England. I use 7 to 8 yards per kilt, and even in the height of summer on this sunny southern shore I can wear an 8 yard black cotton kilt, as long as I don't sit down in the sun.