If a breeze upends your cotton/poly X-Kilt, think of what it will do to a ripstop nylon one.

You could consider making an X-Kilt out of bull denim, which is essentially a polyester denim, in various colors. However, that stuff is pretty beefy and you will have to be careful sewing through multiple layers in places like the waistband.

Again the ABSOLUTE coolest, most wonderful contemporary kilt fabric I've ever used is the Hemp and recycled polyester blend that I've posted about several times. Wonderful, wonderful stuff. It's not $4 a yard, that's for sure, but when you're done, you have a kilt that is washable and has a swing comparable to a wool kilt.