Derek, you might consider making the waist to expand to a 32 inch waist, thus making an apron that will expand, with an extra hidden pleat that can be let out as you expand with age. This is the voice of experience from a 61 year old that once (just three years ago) had a 32 inch waist that swelled to the current 34+ inch waist.

A girth will grow, even if the exercise (work lifestyle) should prevent the "spare tire" of age.

I think the kilt you are building will become a favorite for a long time and even a go-to choice.

Forgive any uncomfortable mentions of unwanted weight gain, the facts of aging are cropping up on me and all of us will have to deal with these small details.