If that is the area over top of the hitch, looking from the rear, mine was open as well. When I had the insulation blown in, I stuffed fiberglass batting into the area. If you have insulation blown in, you can remove the two hitch lights and blow some in there, as well. Also, where the front fiberglass curves down under the front storage/battery compartments at the bottom (underneath), there was a ~2 inch gap, all the way across, between the cap and the frame; and no it was not screwed or glued. I used almost 2 cans of expansion foam to fill that area.
2017 36RSSB3--2012 F350 KR, LWB, SRW, bilsteins, 1 1/4" anti sway bar, bilstein steering stabilizer, airlift air bags, on board wireless compressor, 295/20 tires at 4080 pounds each, oversized finned aluminum diff and trannie pans, all synthetic fluids; KEM, Tyrant, and PIP tunes with SCT X4 programming, G37 2017 Turbo, ATS Stage I towing transmission and torque convertor, Edge CTS gauge monitoring.