Antenna ground planes for composite aircraft

I'm using pre-made di-pole antennas for the comm radios in my Glasair.
But I need to install antennas for the transponder and marker beacon.
I've found some pre-made dilpole marker antennas, but for the
transponder, I'm using one of the blade type, and for that one, I'll
need a ground plane. I'm installing it on the belly panel. What I'm
wondering is how big to make the diameter of the ground plane, and how
to make contact with it to the blade type transponder antenna. I would
assume it somehow needs to connect to the outer portion of the BNC
connector? And what about the GPS antenna, does it need a ground
plane? I've seen lots of pictures of ground planes made with strips of
copper foil tape radiating from the center. Which looks adequate, and
should work as well as a big piece of solid copper foil, which I have
no idea where to purchase. I also read an article by Bob Archer about
di-poles. He said if the dipole has a little black box in the middle
of it, not to use it, as that contains ferrite beads which greatly
reduce it's effectivness. Unfortunately I already have that type
burried inside the leading edge of my verticle stabilizer.
Rich