Rear plastic drain panel repair

[rodo]

I could not find a thread: has anyone attempted to repair their plastic panels on the rear? (This is the plastic piece that fits over each rear wheel well and has a drain at the bottom.) PO cut holes for speakers; now there are big holes. Whole "front" of the plastic, the flat part, can just come off if I tug. Seems like one could cut a piece of...some kind of plastic and then...glue (?) it in place. Anyone come up with a nifty fix for these?