I've made six kilts now and the only closure I have ever made has been velcro. For me, I cannot imagine spending all that time on a buckles that will be hidden by my belt anyway.
I use a 3 inch strip on the left of the apron and a 3 inch strip on the right.
So far it has worked out very well and has given me more than enough room for what fluctuations I may have in my waist. Of course I wear a belt and that helps hold on the kilt too.
I judge 3 inches on each side as a good compromise between usefullness and economy. Velcro can be pricy.