How about using a bit of dark green or blue fabric to make the under apron? Chose a shade slightly darker than the fabric you have and anyone glimpsing it will assume it is the tartan seen in shadow.

If you make the apron the recomended inch (I think it is) larger so it covers the under apron completely it should hide the addition and you should then have enough extra fabric to make good sized under apron pleats, which I think are a good thing, as they open to reduce the possiblity of the aprons being pulled apart.

Anne the Pleater :ootd: