One of the most wonderful humiliations a mistress can inflict on her slave is to "lock" his penis in a chastity cage. Not only does this massively increase the slave's horniness due to the lack of opportunity to satisfy himself, no, he also loses control over his manhood.
It is especially humiliating for the slave when the mistress then wears the key on a necklace or anklet to show it in public.
Real fetishists prefer cages made of metal, the smaller the meaner, and there are also various goddies such as PA, piercing or a insight dillator to make the cage "almost" escape-proof... and to stimulate the lucky slave even more.
Once you've been locked up in such a small cage for weeks, the word "horny" takes on a whole new meaning.