Note that when I asked, Van's does NOT recommend mounting any antennas on the fiberglass cabin for structural reasons. So I have mounted my antennas on the aluminum skin behind the fiberglass cabin top.
I have mounted 2 GPS antennas, 1 Comm antenna, and the ELT antenna in the space aft of the fiberglass cabin top while meeting the recommended spacing between antennas. I then have a second Comm antenna mounted on the belly aft of the baggage bulkhead, the VOR/ILS whiskers at the top of the vertical stabilizer, and Xpndr antenna mounted under the front seats near the tunnel.
Of course, as an experimental aircraft, you can mount the antennas on the fiberglass top if you wish, even if Van's doesn't recommend it.