Modern fly plaids are a smaller derivative of the upper part. I am not sure how it would be held in place, the older detattched upper part that is (though a reenactor may know more than I in this matter). Generally, I think they just adopted a wool or jacket, which then further pushed the replacement of the belted plaid by the philabeg and the modern stitched kilt.
I KNOW that is what happened in the Brit Army. It is reasonably safe to assume the same in the civi-world too.

A Piper's plaid is also another modern form, though held more like a folded blanket than the upper part of a belted plaid.

So, Oatmeal Savage, have we satisfied your curiousity, or shall we continue?