I didn't archive the link to it, but I do remember seeing a pulley designed for flatted extruder motor shafts, but the inner diameter would have to print perfect to work. BTW - you did properly add an M3 nut to the slot for it in the pulley, right? The words "incredibly loose and just wiggles around" makes me wonder this.

EDIT: Well, at least some of the pulley designs accept a nut to hold the grub screw. I'm assuming the one you're using does as well.