The deck behind the headrest is actually the top of the fuselage. The canopy slides on three rails, one on each side of the cockpit and one on the top of the fuselage.
Early Skyraiders did not have the boot.
It appears that it was added to ensure that a leak from the mechanism would not be sprayed into the cockpit enclosure. The boot was not always in place.
The headrest that was attached to the armor plate could be tilted forward. Some other pictures;