It's hard to add much to Steve Ginter's excellent monograph on the FJ-3 (http://www.ginterbooks.com/NAVAL/NF88.htm) but while pictures of the area under the canopy are included, some modelers (well, at least one) have asked for more detail.
I was also surprised in researching the question to discover that the ADF antenna system changed between the early FJ-2 and the FJ-3. The early FJ-2 had the clear dome mounted aft of the armor plate and a sense antenna on the inner surface of the sliding canopy; the dome was replaced by a wire loop on the FJ-3 with no sense antenna on the canopy.
There is a picture of the FJ-3 antenna from page 190 of the Ginter monograph.
As for the rest of the hardware under the canopy aft of the headrest, note that there was a shear web between the sides of the canopy with a couple of items located on it.
The screw jack that opened the canopy was mounted on the fuselage deck below the shear web along with some other odds and ends. When the canopy was closed, the area was greatly simplified from a modeling standpoint.