Another vote for the bath mat on the wall to drip dry with the window open(preventing mould growth). We bought a caddy from tkmax of all places and amongst what I like to call the jumble sale thry had a really strong suction cup with a storage caddie and i mean it holds all the crap eg toys, sponges, shampoo, wash and bubble bath. Found it ace tbh x
A suction cup that has a hook on it, and a mesh bag with a draw string would probably do the trick :) In terms of the bath mat we peel it up and stick it to the wall so it can drip dry , prevents the mould growing on the bottom as quick / at all x