I saw one once...

A slave is suspended in the air by his wrists. There are ankle cuffs on him. The ankle cuffs are connected to rods that follow the inside of his legs up near his ass. And the top, they are hinged so the rods are like an upside down V shape. As the slave spreads his legs, the apex of the rods moves away from his body. Putting his legs together, or just letting them hang freely, causes the apex to move closer to him.

Aaaand... of course there's a HUGE, thick, tapered dildo on the apex. As long as he holds his legs apart, the ass violation will be minimal.

But gravity is a bitch, and even the strongest slave can only hold his legs apart like that for so long before he has to just let them hang so his muscles can rest.

Which causes the insane dildo to penetrate his bitch hole deeply, causing him discomfort from ass raping.