I live in hot and humid Florida and have found that USAK's pv Casual kilt is a good choice for summer. The pv (from Marton Mills) Rocky uses breathes better than wool and is machine washable. USAK's Casual is also very reasonably priced for a well-made custom kilt.

Another factor to consider is the amount of material in a kilt. A 4-5 yard kilt is going to be cooler than a 7-8 yard kilt.