For me as a DM, druids can wear any light armour or hide, no problem. If they have armour proficiency (e.g. from being a Dwarf) then they can use that armour -- it doesn't prevent casting or getting spells, for instance. If it came up (it hasn't) I would allow it with multiclassing too. Unless manufacvtured in-game (i.e. not bought in a market), I wouldn't allow/worry about chitin. bark, whatever alternatives.
As a player, I despise the limitation which feels artificial and under-rationalized, a legacy holdover. So I work around it, either with a human taking magic initiate so they can cast Mage Armor, or a Lizard Folk, or whatever.