He might have had very warm feet! The front turret, on the (relatively) rare times it needed to be used, would be manned by the radio operator (WOP/AG = Wireless Operator/Air Gunner), who sat in the warmest place in the aircraft, where the hot air inlet was!
Best way to blank-off that 'hatch' would be to form a curved 'wall' across the entire height and width, to meet the edges of the turret transparency. The real turret was curved at the rear, with a sort of semi-dished rear wall of aluminium. The pic below might help a bit, but if you need a better reference, let me know and I'll dig out something.