What is the highest armour class possible for an Ancient Green dragon?

This posts asks what the highest armour class may be for humanoids. Similar to this question, i would like to know what the highest AC would be for a monster. Narrowing this down, let us look at improvement techniques possible for an ancient Green dragon. As this is a BBED (‘Big Bad Evil Dragon’), they most likely have a slew of minions, guardians and servants – so please feel free to use any options that exist (in 5e) that may be spells, minions, a spare shield guardian, or anything else you like.

For stats on the ancient Green dragon, has a base dexterity of 12 and a base AC of 21. I do not know if it could gain NPC levels of monk or barbarian, nor what use that would have. We can assume this dragon has the ‘optional’ spell casting abilities. According to old (possibly 3.5e) Forgotten Realms material these creatures are a minimum of 800 years old at ‘ancient’ and may live until past 1,200 years or more. They seem to have access to a lot of magic items. These things are gargantuan, so i have no idea, for example, what size shield they would need – or even if a shield works for them. I don’t even know how big ‘cover’ would need to be in order to offer 3/4 protection. Would a Cloak of Protection even fit? I have specifically left this quite open as i would greatly value any and all possibilities that Stack Exchange has to offer.

Let me know if this question is specific enough. The original (above) was seemingly less specific, but Stack Exchange seemed to be enjoying a different mindset at that time.

Question Recap: What is the best armour class possible for a 5e ancient Green dragon – assuming any & all options are available to it.