This is a cool image. I would probably make it be +1 or +2 armor (dependent on your party's level), as the AC of a beholder is 18 (natural armor, which makes its scales either the equivalent of plain plate armor, or +2 scale mail armor).A similar idea that is official in 5e is Dragonscale mail, which is very rare +1 scale mail armor that gives the attuned wearer advantage on saving throws against Draconic Presence and Breath Weapon effects, and resistance to a draconic-damage types, and the ability to sense dragons within 30 miles of you as an action once a day.
So, if you were to take this and change it a bit, I would make Beholder-Skin Armor a set of Very Rare scale mail armor that requires attunement, granting a +2 AC to the wearer, advantage on saving throws against Beholder Eye-Rays, and the ability to get a beholder's 20-foot hover speed for 1 minute.
(I also like the ideas of eye-stalk based items, too. Eberron: Rising from the Last War actually has a similar magic item, Belashyrra's Beholder Crown.)
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