DIY - travel flats (the flattest)
ingredients: one pair of ballet flats, one reasonably competent shoe repair place
thin rubber shoe tread that you can cut to size, serious glue, thin fabric or leather patch. glue sole to bottom, glue patch to inside above where toes go to reduce the effect of the soft leather molding to your toes.
(top pic - new shoes; bottom pic - finished product)
i happen to choose to add an arch support to mine.
makes for the flattest, most packable shoes ever (questionable durability...)