antenna ground planes

I just bought a fiberglass homebuilt project with a conventional com
antenna mounted inside the leading edge of the vertical stab but it
doesn't have a ground plane yet. The instructions call for a 24in by
24in ground plane but there isn't that much room right there at the
base of the tail. Any suggestions as to what would work well? I
imagine there is info out there somewhere but am not sure where to
start looking.

many thanks,
steve