How far HMS Eagle could be upgraded for air defense

How limited was HMS Eagle's ability to receive stronger defensive anti-aircraft armament?

The main limit is how much money the Royal Navy is willing to spend; Eagle has usable space for more light weapons, but a full modernization may be less sensible than a new carrier.

Eagle has room for additional light anti-aircraft weapons after removing some existing guns, though not for a wholly new heavy battery. Carrier defense worked in layers: fighters disrupted strikes, heavier guns broke formations up further, and lighter weapons engaged the aircraft that pressed home. Eagle offers more deck space for open or shielded mounts than the smaller Hermes, so physical room is not the decisive obstacle. Defensive success includes frightening attackers into releasing early or turning away, rather than merely shooting aircraft down. The later comparison with torpedoes, rockets, and other stand-off weapons shows how greater release distance reduces the number of pilots who must press through the innermost layer.