Another choice: ¼-½” aluminum strip, wide enough to reach across two of the mounting bolts. Add 2 nuts to hold it, and you can mount to that plate with no holes in the fiberglass… I’ve seen this done by several different people over the years.
As a note, you will have issues with corrosion if you lock the battery compartment with the chargers plugged in. All that hydrogen has to escape somehow, along with other gasses released during charging.