Any item can potentially be made slotless; that's what Ioun Stones are, just with fluff. Instead of a floating stone, make it a belt buckle or an earring or a strip of cloth you wrap around your arm. Costs (bonus^2)*2 to make it slotless, or you could just make it a weird slot (look at my leggings of charisma!) for 50% base rather than double.

The best thing though would just to get a cloak of resistance and charisma or the like.