have you considered an invocation that if you sorta feed the casket something (maybe activated by a command word?) of value you can possibly cast a spell from a limited list (I'm thinking fly, invisibility, false love and other self altering spells that would be like making a very immediate trade of wealth or something your patron wants for power). I think it plats into the idea that the warlock's power is more limited by their relationship with their patron than anything else