There already is a box pleat in a Traditional Knife Pleated kilt. It is just to the right edge of the under apron.

We call it the reverse pleat. It is there for just the reasons you state.

It's brother on the other side is called the deep pleat.

They allow for more fabric at the apron edges to allow you to prevent modest problems, and if you back them up with hidden pleats you have extra fabric for re-sizing the kilt if necessary.