I'm also looking for more ways of increasing my height. usually I make my own inserts by buying cheap foam sandal platforms and carving at them until they can fit into my shoes while still maintaining the heel height. I can get up to 4" of extra height with this, but it's not always comfortable and depending on the type of shoes the character wears, it can make your ankles look unusually big, haha.
Occasionally I will "make" my own shoes, by taking an old shoe base, adding the heel, and then building the shoe around it out of canvas, interfacing, or other stiff materials and covering the whole thing with appropriate fabric.
Shoe lifts look like a relatively simple and comfortable solution. But can you stack them to get more height?