Personally i think rainskirts are a better idea for warm and wet conditions, and they are super easy to make. They aerate and cool better than shorts, and it's not even necessary to use WPB fabric in that case--hence some light and cheap sil nylon or the like works fine. I do like tyvek rainskirts though (which is somewhat breathable--though i have no idea about the CFM levels). I've thought of making a cuben fiber rainskirt--it's the one thing in C.F. that i might be able to afford.
If you already have a Kilt, it's super easy to make--use the Kilt as the "pattern" to cut out the fabric, attach some velcro (either gluing or sewing) to two points on the fabric and voila.